If you are planning on accumulating a large library for one or the other, one factor to keep in mind is that game files on the 360 (assuming you don't install the game) tend to be much smaller than on PS3.
Comparing the back of the boxes can be a real eye-opener - nearly a gigabyte required on PS3, with the 360 version clocking in at 4 or 5 megabytes - things like that. I believe that's to make up for the slower read speed of the Blu-Ray drive.
If you are only going to have a dozen games, that may not matter to you (and obviously it varies from game to game), but it can be a deciding factor when all else is equal if nothing else.
I can use memory cards for 360 right?
I know you can put game saves on a flash drive, but beyond that I'm not sure what your storage options are. If you are just using it for disc-based games, even one of the smaller 40gb hard drives will be enough for hundreds of games. I know, because that's what I have - ~120 game files and saves, and all the patches, with free storage remaining at about ~28gb.
Obviously, it will depend on which games you have, but overall, game files/saves/patches are rather small. I personally have never bothered with flash drives, and the one time I've wanted to move from one console to another, it's trivial to take the hard drive out on a 360.
Bit of a tangent, but you might want to know: The 360 has the hard drive in a proprietary enclosure, so you need an official drive, whereas the PS3 just uses standard 2.5" drives. So you could put in a bigger drive if you wanted to, or even an SSD, if you wanted to see slightly improved performance.