For me it came down to what "exclusives" the systems have, for me the X360 games just appeal to me more; Halo3 was very tempting, but BioShock sealed the deal for me.
IMHO, PS3 exclusive are mostly "kids' games". The only compelling exception is GTA. I will buy one for my kids when the price vs. titles ratio reaches an attractive level.
Lastly, for "non-exclusives" you need to do some research. Some of them are designed for one system, and the ported very poorly to the other system. So you need to consider that factor too.
IMHO, PS3 exclusive are mostly "kids' games". The only compelling exception is GTA. I will buy one for my kids when the price vs. titles ratio reaches an attractive level.
Lastly, for "non-exclusives" you need to do some research. Some of them are designed for one system, and the ported very poorly to the other system. So you need to consider that factor too.