My choice would be Dreamcast. Collector item! (Soul calibur 1, Shenmue 1 & 2 not 3 :(, Powerstone 1 & 2, ToyCommander, Eternal arcadia).
Except if you want to play Halo 1 & 2, then go for Xbox. But Xbox is not really a essential collector console.
For both Games & Collector factor Dreamcast > Xbox, but Xbox has halo 1 & 2 which can be a deciding factor for many. I personnaly really liked Halo 1 but I loved many Dreamcast games!







