As far as Sonic Adventure 2 goes...you play just as much sonic-style gameplay in that, if not more, than you do in Adventure.

And yes, playing as Shadow is essentially playing as sonic.

Sonic Adventure 1 right now is pretty much just a so-bad-its-good game. It has its good moments, but for every part where you're like, "yeah, this is awesome" there's two that go "THIS SUCKS"
The cutscenes Oh god!

Well I think the original Sonic Adventure is the superior one, it was a groundbreaking game in 1999, and it's still amazing. Even Big's level can be fun in small doses, the rest is just amazing, especially the final fight with Chaos. It's a shame the sequels overused the seemingly invincible monster in the sequels...