If you are thinking a big reason you want the PS3 is because you might want to play your PS2 games, think again. If the PS3 supports them at all, there will probably be significant graphics and sound issues.
http://uk.ps3.ign.com/articles/745/745439p1.html
Much as the Playstation 2 Slimkit will have performance issues with older PS2 games, the PS3 probably will never be fully functional with last gen titles.
At this point, the answer is 360. Better games, better functionality, better online access. It doesn't have 100% backwards compatability either, but it doesn't matter because so many new titles are out.
In a year the library of PS3 games may have something worthwhile. For now, the 360 is king.
(and cheaper)