Both are fantastic, its such a tough call. SA1 has a more enjoyable Sonic thanks to more open ended level design but I do appreciate the arcadey high score precision that SA2 demands. Whilst SA2 did downgrade the radar, I'd say the treasure hunting stages for Knuckles/Rouge are way more fun in SA2 than they were in 1. But then you get to play a traditional Tails in SA1 and he's awesome! Gamma and the mech stages in SA2 are kinda whatever for me, I don't hate them or love them. At least with SA1 the Gamma section is over in like half an hour and it doesn't have that ungodly ear piercing lock on ring that SA2 has.
Many people bash Amy and Big but I think both of the campaigns are short and ultimately solid, especially Amy's.
But when it comes to bonus content going for A ranks, playing alternate missions and delving into the Chao Garden is infinitely more fun in SA2 than in any version of SA1.
Musicwise both are peak, but I do appreciate SA1's greater variety in genre and style.
Whilst I conceptually like SA1 more as an overall package, the dealbreaker for me is that the overworld - while cool - just doesn't have enough going on to justify itself. Its small, there isn't much of anything to find and it just serves as tiny padding between actual levels. If there was just more to see and do I'd probably give it the edge, but thanks to SA2's pacing for a campaign playthrough being so damn strong I ultimately think I give it to that game. You may not have non-mech Tails or Amy, but everything else makes up for it.