Here's a simple way to look at it.
The following things need to be a major asset to you:
Blu Ray Capability
Non-Proprietary Hardware (Any Compatible HDD and BT Headset works)
A social gaming environment (Home, which is still in beta and the jury is far from being in about)
SCE-based Exclusives (Ratchet & Klank, MGS, God Of War)
A Multimedia file player (This is almost negligible but to be fair, I've found playing an XviD video with DD sound is easier on a PS3 than a 360)
Dual Shock Controllers with better pads for fighting games
PSN PSX Downloads (which is almost a joke because you can probably find a bunch of the games for < $5 at your local game reseller)
Console based web browsing (which in reality is only nice for watching Youtube videos on your big television instead of your small computer screen)
If you're looking for a system for gaming ONLY and you already have a 360, the PS3 is not the wisest of investments right now. Nearly every game that comes out for it also comes out for the 360 with comparable graphics and sound. In fact, on more than a few occasions the 360 pulls off various graphics and sounds better than the PS3 does. I got a PS3 when the MGS4 bundle came out, and I only have that and LittleBigPlanet for it, because I have a 360 and will often get a 360 version of a multi-platform release for the achievements. Granted, PS3 now has Trophies, but they're not available across the board and sometimes are released later and force you to repeat your Trophy "acts" all over again.
Also, as far as the free online gaming goes, remember that while it is free, it's not as organized as XBL is since they leave it up to developers of the games as opposed to using a backbone like XBL does. In addition, if you aren't doing a lot of online gaming now and paying for it, what's to say that you're going to do more when you DON'T have to pay?
All that said, if you're willing to dive into the Blu Ray world and like all the other added features that the PS3 has that are absent from the 360, then it is worth the investment.