College is Abit different than the NFL where talent level is alot closer and you have a salary cap than college teams who can recruit the best players and sometimes have NFL talent going against guys who will be in sales next year.
Look at urban Meyer in the NFL vs college
This is the biggest factor by far. There's 0 parity in college football. Still I think if you're objective and purely looking at from a coaching pov and all that it entails. I think it's possible for a college coach to be better. You're dealing with stuck up 18 yo who don't even have to ask for a trade. They can jump into the portal. Anyway NFL is absolutely harder but I think it's possible. Bill has such a resume though, it's so tough to put anyone next to him. Saban is definitely as good imo.
Saban has won it all with different teams and a multitude of QBs. He's got a .799 winning record and the same NFL winning % as Belichick w/o Tom Brady.
Saban >>>> Bill
It's relevant for sure in this case (strictly comparing him against Bill and post-Brady) but a bunch of college coaches have won with different QB's.
Reasons to argue Saban ...
It's harder to coach and develop younger players. Sure he gets the best but he also churns out guys like Mac Jones (15th overall) , (Josh Jacobs 1st round nobody knew his name before Saban touched him) , James Carpenter (Who, exactly, had no business going round one. In fact there's a video of Saban saying (James Carpenter round one??? Or something like that draft night) There's others I can mention - Ryan Anderson, TJ Yeldon. Dealing with guys like Reuben Foster who couldn't read at a high school level.
Whether they come in at 5 stars or 3 (a lot of that is overblown anyway) he churns out talent that dominate the draft.
Saban might be a better DB coach. There's really not many guys on the planet that can touch him there and from the sound of things he looks to be more involved overall. I think most would agree with coaching defensive backs.
He's as or more flexible imo. If you've watched Bama you know how many style they've won with. Power, spread, RPO - NFL has been stealing from college so those guys are doing it first. He wins with defense one year, offense the next.
I think every player from every recruiting class that stayed 4 years won a championship, maybe 3 years, I forget. He's also won in 3 separate decades.
He's built up multiple college programs into contenders.
He definitely has a case for GOAT HC, college or pros imo. Most of the "who's better" debates are boring to me but that's an interesting one.