The interesting thing will be to see where the hardcore, tech diehards go.
This generation it was the Xbox. Next generation, if the PS3 is substantially more powerful (a big if, but it could easily be as big a gap between the XBox & PS2), then it comes down to what advantages Xbox has.
IMO, they are: (this is, of course, without truly being able to compare specs, but assuming that a later system is more powerful)
Xenon Pros
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1) Ease of development
2) XBox Live head-start
3) Microsoft franchises (Halo, PGR, etc)
4) Released first means getting fans to commit early
5) More money to buy exclusives than anyone else
6) Western Developers w and w/o PC ports (Bioware, Lionhead, etc)
PS3 Pros
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1) Most powerful system (likely, but certainly not guaranteed)
2) Blu-Ray Drive (storage, movie player, etc)
3) Sony's Franchises (GT, SOCOM,etc)
4) Eastern Developers (Square, Konami, Namco, etc)
5) Probably the most broad overall support including all developers
Revolution Pros
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1) Nintendo's franchises (Mario, Zelda, Pokemon, etc)
2) Some unique hardware / functionality
3) Could be more powerful than Xbox