After years of waiting and hoping that SEGA would release SA2 for PC (having released SA1) I finally gave in and bought a Dreamcast version. Then I bought the Nintendo GameCube and ever since my Dreamcast has been gathering dust in my room.
Let's break it down.
The graphics. This goes more or less without saying; they're at least on a par with SA1, if not better.
Continuity. This may seem trivial, but it really grated on me in SA1 where you'd see one cutscene from Sonic's point of view, then the same one when playing Tails and the characters would say something slightly different.
The story. Gotta be the best Sonic story of all time!
Level design. I can play the same level over and over again and still find new things. In particular I like the Hard setting (fifth emblem).
Music. Great songs. 'Nuff said.
Difficulty. The levels start easy, like they should, and get progressively harder without too big a leap.
Big the Cat. Finally! Anyone who got frustrated with his levels in SA1 is going to love the cameos he makes here. That cat sure gets himself into some tight situations ;)
Stage Select. A much neater and better way of replaying levels. Also, when you beat Hero or Dark side, you get to watch the cutscenes again (a big plus compared to SA1)
Chao. You can now name them in the kindergarten, classes take a much shorter time to complete and there's the Black Market, none of which was available on the Dreamcast. You can also get your Chao's stats and medical report. At least, not without some serious hassle. The only problem is the redesign of the Dark Chao garden (hey, I liked that cave and the graveyard). And the fact that you can't get the Moon Chao egg.
Camera Angle. While occasionally it's useful to know which way you're supposed to be going, sometimes you WANT to go backwards through the level (right now I'm thinking of various missiles you need to fire). Since every time you try to correct the angle it corrects itself back again, resulting in you going around in circles and often dying, you get used to playing backwards. There's no Free/Auto setting either.
Emerald Radar. (Note: this applies to the radar on any of the 'find it' levels). For some reason, Sega decided to change this. Now it detects the shards one at a time, from left to right. This basically means that you could be right on top of Item 2 or 3 and not even know it. For most of the levels, this is just irritating, but on Rouge's level Security Hall when you're racing the clock, it gives a whole new meaning to the word 'frustration'.
Hidden Upgrades. Not that much of a problem - the levels are arranged so you can't go on without the vital ones - but it's still irritating to not have a clue where to go. Mystic Melody is the worst one for this, and it's impossible to get all third rank emblems without it.
AND THE JUST PLAIN MISC:
Linear Gameplay. I honestly can't make up my mind whether this is good or bad, so I'll stick it in here. You get a far greater sense of continuity compared to SA1, but if you get stuck on a particular level, you only have one other character you can play.
Inadvertent Homing Attack. I don't know if this is just my controller or the Gamecube version, but sometimes when you push the jump button once, Sonic or Shadow decide to do a homing attack instead. This isn't too much of a problem, but on the Egg Golem boss with Sonic it's incredibly frustrating. It took me fifteen minutes to beat that one on the Gamecube, as opposed to only a few on the Dreamcast. If memory serves, it went something like:
SONIC: (Jump...jump...ju-HOMING ATTACK! Splat)
Over and over and over again.
Lack Of Adventure Fields. Again, I can't make up my mind if this is good or bad. I loved the adventure fields in SA1, but they probably wouldn't fit in here.
So the burning question is, do you buy this or the Dreamcast version? If you're never going to do anything with Chao, then in terms of gameplay it doesn't matter which you get. If you want to do anything with Chao at all besides pet and play with them, you need this version.