Dreamcast was great for Phantasy Star Online, 2D fighter, 3D fighters, Sonic Adventure titles were good at the time, Shenmue 1 and 2, Skies of Arcadia, amd other things.
I played very things on Xbox, like Halo, Doom 3, Half Life 2, Max Payne games, Fable, CoD titles, and numerous Sega games.
Gamecube was good for Dreamcast ports and Nintendo stuff, but I didnt spend much time with it.
I'm gonna put Xbox on top. I have nostalgia for Dreamcast but Gamecube is strong if you like Nintendo stuff.
A lot of Xbox games are better on PC now. But emulation is better than a real Gamecube at this point.