I think the decision making is very easy. It always boils down to money. If you have lots of money, get both systems, there is enough differences to justify it. If you have little money, then you want to spend as little on the hardware, and the rest on software. For example, looking at crossplatform games only, if you were a fan if FF, GTA4, CoD4, RE, etc, etc, and that were the game(s) you 'had to have', then in order to play that game you would spend $400 for a PS3, and $200 for an Xbox (plus the cost for games for each system). The game(s) will play the same on both systems, and if you decided that you wanted a harddrive, MS is offering a HD upgarade program for $30 (20G HDD + 3 months live) for Arcade users (thus you can copy the ENTIRE game to the hard drive, and the game will run smoother, quieter, and load times will be faster).
In all fairness, both systems have great exclusives, and unless you really have to have a specific exclusive, do not use that as justification. There are just too many good games on both systems that you will not have time to play them.
The other thing is that there are many more games on the Xbox360, and many AAA games from the last 2-3 years that can be had for super cheap (from Mass Effect, to 1st Gears, to Halo3, Oblivion, forza, PGR, R6V, GRAW, Assassin Creed, bioshock, and many others that have scored >85% on metacritic/gamerscorerankings.com) that you can get these games for $10-$20 used-->new and never have to buy a 'new game' ever as you will not have time to catch with the newer games. Because Xbox games usually sells more than PS3 (and especially if you live in N America), the used game market is much larger on the Xbox, and thus more competitive, and thus cheaper used games.