I've owned all 3
SC 1 - Great. DC version visualy is better but I prefer playing the jamma build. Its also rather cheap now.
SC 2 - Very simular YET, Arcade Conquest & Console Conquest mode are nothing alike. Also the arcade version gives you much more options then any previous Arcade fighter (time trials, conquest, story mode, Practice). Yet I don't belive there is unlockable characters (i've yet to get any) yet no anoying BS add in characters (link, spawn, ...) which i love since I hated the exclusive characters.
My Personal Opion.
Dc version is better then arcade version of Soul Calibur 1.
Soul Calibur 2 is better on arcades yet lacks unlockable characters (which happen to be 2 that i like the most). Visualy xbox has more polygons in the enviroment with little things like particles and blades of grass but its pretty even. The arcade version texturing is just as good as the xbox build.
If you have a 31Khz Arcade monitor then it's posible to make SC2 look much better then on a standard TV. Yet if you have a HD Tv and run the xbox build in 480P, then they would be equal.
Problem? Soul calibur 2 is freaking expensive as hell. $600-$800 as it's still a pretty good money maker.