Overall the Xbox had the (slight) edge (textures/effects/a lot of unified memory) visible in most multiplats games like splinter cell notably.
But Gamecube was a monster in pure polygons processed with the 2 Rogue Squadron games and it's 15-20 millions polygon/sec displayed. I think not even one Wii game were prettier than the Rogue Squadron games.
But the split memory architecture of the Gamecube with main ram/Vram was already a problem for developers. But Gamecube memory was very fast, it just hadn't enough of it (24 main ram, 3 MB for Vram I think). Even 24MB for main ram was not enough when PS2 had 32MB and XBox 64MB.
So I would say Xbox but only barely.







