I own DC, Xbox, GC and PS2.
SC2 looks definitely better than SC1, the improvements might not be noticeable when watch SC2 on itself but next to the DC version it is very noticeable (vivid colours, more detail). However the DC version is very, very good. In fact if you would make a Fighter cabinet, pick up a DC hack the pad and you're done.
I've tried all three SC2 versions side-by-side to decide what version to buy. All look the same. In fact: most of the time you couldn't tell what version was running other than figure it out by pushing the controller buttons to see to which controller the game reacted.
In the end it came down to 2 things: bonus character preference, controller preference. I liked the PS2 controller the most (it the Xo / [] layout is similar to DC's ABXY layout, however I didn't like the bonus character. I bought the GC version because I liked the bonus character (Link) and the controller worked fine. (controller pref was 1 PS2, 2 GC, 3 Xbox - char preference: Xbox, GC, PS2).