Whatever system has more of the games you like to play.
The end.
I wouldn't be concerned too much about the RRoD, as you're covered under a 3yr warranty for that type thing, and new 360's seem to be much less prone to trouble.
I would say, though, that you should *really* look into downloadable content. There are some great games for both PS3 and 360 that you can only download.
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Really, there's only one true way to go about it -- Look at the top games for each system and see what you like.
Personally, there were LOTS of other factors that made me choose PS3 over my 360 (I gave my 360 to brother), like free online play, quieter system, Blu-Ray, etc.
If you only want games, take a long look at the games for each system and purchase accordingly. You really can't go wrong with either selection.
You might also want to look at 2008's upcoming games. 2007 was a great year for Xbox releases, and 2008 looks to be PS3's stronger year (last year was sorta weak).