As a person that owns both...I prefer PS3. If the game is multi-platform, I get the PS3 version. KHking already has a decent list as to the reasons but I will throw in a little bit.
As for the charge packs for the 360...yes they work, but are they as convenient, no. You have to spend extra money in order to have them where as the PS3 controllers just come that way.
PS3(Blu-Ray) discs can hold a lot more data than the HD discs used for 360 games. In several cases, there has been content cut on 360 versions of games to save space...not to mention, large games have 3+ discs where the PS3 gets it done with only one. I have had issues with multiple disc games being glitchy & sometimes freezing on the 360. Though this could also be hardware related, I am not the only person that has had these issues.
As far as the graphics go, both systems can perform admirably. However, the Blu-Ray technology tends to make games more crisp & polished than the 360 versions. I have seen side by side comparisons on a few games & I can see a difference. It may not be enough to bother most people or even be noticable for that matter, but it is there.
Overall, they are both good systems. For me, the pros to PS3 far outweigh those on the 360...but once again, I do own both. The reason for this is simple..."exclusive" titles. There are titles that I like & want to play/own on the 360 that are not offered on the PS3...not too many to be honest, but still enough for me to own one. In all honesty, if I could get all of those titles on my PS3, I would sell the 360 in a heartbeat.