Well, I do own both, so I suppose I should throw in my two cents worth.
First of all, if you already have a ton of games for your 360, it seems a little silly to not play them, or sell them off for a fraction of what they're worth. For that alone I'd probably just get a new 360 in your situation. Graphics-wise, there's very little difference, and so many games are going multi-system anyway.
Although I will say that the PS3 has at least caught up on the exclusive games war, if not surpassed the 360. Only games I care about that are XBox exclusives are
Halo,
Mass Effect 1 and 2,
Dead or Alive 4, and
Tales of Vesperia. PS3 has
Demon's Souls (the reason I bought a PS3 in the first place),
Uncharted,
inFamous,
Metal Gear Solid 4,
Little Big Planet,
Valkyria Chronicles, and
Disgaea 3. Of course, there are plenty more exclusives on either side, but those are just the ones I own and care about, and right now the PS3 is winning.
Also, the PS3 hardware is more reliable. Always has been. And with the new slim models, although they haven't really been around long enough to assign them an average lifespan, they run cooler and quieter, so I'm thinking they might last even longer than the older models. We'll see. Of course, the XBox 360 has that ridiculously awesome 3-year warranty because Microsoft knows they foisted phony hardware on us. AND there's the fact that when the 360 fails, your data is generally safe... you just detach the top and buy another machine. With the PS3, the HD is not detachable, and there's decent odds that when it does die, your data goes too. Not cool. Just be sure to keep backups, the PS3 does have a utility for that.
There's also the fact that the PS3 can just *do* a lot more. First and foremost is obviously that it's a Blu-Ray player. Plus you can easily network with your PC, steam movies, customize your background, organize pictures, etc.
I also like that the PS3's online play is free. It's not like Microsoft charges that much... you can usually find a deal for like $40 a year for LIVE, but I just have a somewhat religious aversion to paying ongoing fees for something like that. It would make me feel like I have to make use of it as long as I'm paying for it, and the last thing I need is to feel pressure to play my video games. I goof off enough as it is.
In any case, if you were starting out from nothing, I'd go with the PS3 over the 360. But if you've got a bunch of games and accessories for one and not the other, you may as well stick with what you have, unless there's some games for the other system that you've REALLY been eyeing...