I like Days of Thunder more than Top Gun. There I said it. Yeah, that movie's core can be summed up as "Top Gun with NASCAR" but I genuinely feel it does the Cruise formula better. How so? Well, let's compare these films' aspects categorically, starting with.....
The Cruise: Tom Cruise could never sell me as a cool guy. He plays cool to such an extent that it comes off as a facade. The best Cruise performances utilize this to their favor by focusing on him either playing a dork (Edge of Tomorrow) or playing a jackass whose "coolness" actually grates on others (Rain Man), often involving some sort of journey that requires him to develop into the cool guy he is usually hired to play. Top Gun's Maverick easily falls into the "regular cool guy category" whose only real arc involves something about dead pilots without any real weaknesses of his own. He doesn't really seem to overcome anything by the end, and feels like the same character at the end without much sense of development. Days of Thunder's Cole? Well, let's look at this conversation between him and Robert Duvall's Harry, his manager:
Harry: "You've got to tell us what's going on with the car."
Cole: "You want to change the way I drive. Set up the car so I don't have to change."
Harry: "Tell me how."
Cole: "What do you want to know?"
Harry: "Is she running loose or tight? A turn here, take some wedge out, we'll win some races."
Cole: "I can't do that. I don't know what the hell you're talking about."
Harry: "How do you mean that?"
Cole: "I don't know much about cars."
Harry: "Neither does any other driver."
Cole: "No, I really don't know. A turn here? A wedge there? I don't know."
Harry: "How can that be?"
Cole: "They told me to get in a car and drive. I'd like to help out, but I can't. I don't have the vocabulary."
Harry: "Well... we're just going to have to figure one out."
In this dialogue we immediately understand Cruise has a dorky side underneath the cool persona he had been holding up to this point. Another layer Days shows off that Top Gun lacks is arrogance: from his early radio chats with Harry we get that Cole has a serious superiority-inferiority complex, barking back at him whenever a race doesn't go out as planned. And the best part is, SOMEONE ACTUALLY RESPONDS TO THIS unlike where everyone in Top Gun just takes his coolness on face value. Which comes to our next category...
The Love Interest: There is nothing to Kelly McGillis' Charlie. Everything about her feels like a shoehorned Hollywood love interest who could have been removed entirely from the film and nothing would be different. Admittedly Nicole Kidman's Claire isn't much different, showing up, breaking up, and returning in all the same manner. But at the very least she has this line:
Claire: "You and Rowdy have the same sickness, it's called denial and it's probably going to kill you both."
Remember what I said about the best Cruise performances above. Here we see a rational side effect of Cruise's aggressive coolness, arrogance, leading to a breakup that is at least convincing enough to not feel as hopelessly formulaic as the former. And on the topic of arrogance.....
The Rivals, Friends, and Surprise Final Bosses: I'm gonna throw Top Gun a bone here. Val Kilmer's performance as Iceman is nothing short of magnetic. The character he plays is stock to the bone, but he is just mugging with the role so gleefully it actually kinda works. His rivalry has no real stakes however because it just goes from "Maverick is dangerous" to "Maverick is cool cos he killed the surprise final boss." There isn't anything here that Maverick has to conquer to develop in accordance to the plot or as his own character beyond having a celebratory wingman declaration at the end. Anthony Edwards also does a charming job as Goose, playing yet another stock role with enough fervor to elevate what is essentially a plot device whose death fuels Maverick to be the best.... even though he already wants to be the best. And we come to Top Gun's surprise final boss, the Mig. He's really just there so the movie has a big action climax and nothing else.
Now let's look at Days of Thunder's equivalents, starting with the rival who ends up developing into the friend: Michael Rooker's Rowdy Burns. Referring back to Claire's quote from above, Rowdy actually feels like an obstacle for Cole to conquer because he shares a similar obsession with victory, coupled with a matching sense of arrogance that further drives each of them to be on the top of the game. When Quentin Tarantino described this as "a Sergio Leone race car movie," he was looking at this relationship, as the racing that first pits them against each other ironically becomes a catalyst for Rowdy to become the Friend. As beautiful as the volleyball scene is in Top Gun where it gives the characters a great moment to unwind from the plot and pretty much just be characters, Days ingratiates the racing element into these two. When they first get out of the hospital, they are so bent on racing that they do so even in recovery via wheelchair. This goes even further when they use rental cars to race each other when they're supposed to meet together, solidifying the two as friendly rivals who realize they can relate in their ambitions in the midst of their competition. When Rowdy ultimately gets too brain-damaged to race, it only feels natural that Cole continue his legacy because no one truly was closer to him. So takes us to Days' surprise final boss, Cary Elwes' Russ Wheeler. He has more build-up than the Mig, but he likewise is mostly just there to have a final action climax. Okay he does rub Cole once to hint at a rivalry but he lacks enough presence to make as much of an impact as Rowdy. I love Elwes as much as the next guy, but he may have even less to do in this than Kidman. This is probably a weakness of both films, as their final battles both rely on one last threat and then just sign off, but a slight presence is still better than minimal presence, so Days is at least a little better in this category.
The Scott: I can't really criticize Tony Scott for his work on Top Gun, mainly because he has an ability to film action scenes using angles that are visually disorienting yet cohesive, allowing the audience to literally see the tension onscreen, yet is able to build character by basically just letting his actors cut loose and shoot the breeze for a good while (see the "Sicilians are half-n*****" scene from True Romance for example). Really it's his direction that makes Top Gun decently enjoyable despite its stale writing, and Days really does benefit from having a stronger script in comparison. But I will argue that his directorial skills improved in Days in large part to the lessened restraint that comes with having more studio brownie points. Top Gun is one of those things that just feels unnaturally PG-13, from having a clearly sanitized love scene between Maverick and Charlie to the action being delivered in a way one can't really imply much blood/gore in the face of burning wreckage masses. Days of Thunder, on the other hand, has Scott pushing the MPAA rating as much as possible. We get insane crashes which we can only imagine racers nonessential to the plot had their bones cracked to bits, Sweet N' Low packets rubbing against as much of Nicole Kidman's thighs as possible (it's a less tame love scene to say the least, despite a similar PG-13 rating), resulting in a more raw experience that feels more like a Tony Scott film and less like a Jerry Bruckheimer & Don Simpson production. Tony is a filmmaker who prided himself on mindful, deliberate excess, and that lessened restraint for Days just plays more to his strengths.
The only remaining category to which I can really use to VS. these films with is music, to which I'll admit Top Gun has the edge because Harold Faltermeyer basically was 80s sound where Days has Hans Zimmer playing discount Faltermeyer. Also, Danger Zone. With the right sense of freedom and an effective enough screenplay, formulas can be made engaging in spite of their predictability. Days of Thunder is an underrated proof of this.