I loved playing both consoles so hard pick..
Dreamcast had Jet Set Radio, Crazy Taxi, DOA 2, Powerstone, Shenmue, Quake 3, Sonic Adventure, RE: Code Veronica, Phantasy Star Online and Skies of Arcadia
Xbox. Had so much fun with Halo and DOA 3 multiplayer fun with friends. Also JSRF, Fable, The Chronicles of Riddick, Splinter Cell.
If I had to choose one though.. I'm slightly edging towards Dreamcast. Pure quality arcade games and excellent adventure games it was a shame it fell on its knees. What an awesome console with so many quality titles for its (albeit short) lifetime.