After the last generation ended, I was a PC gamer for several years. After consoles dropped down to what I thought was an acceptable price, I looked into all of them. I had no bias, so I think I was pretty fair in my evaluation, relative to what I was looking for.
What decided me on the PS3 over the 360 was 1) I'm still a PC gamers, and at that point, more of the 360's games were on PC. Keep in mind that Alan Wake had been originally confirmed for a PC release, which was a minor factor. 2) the PC is my primary online gaming machine, but I still wanted the freedom to play online with my console at my desecration without having to pay a fee. 3) honestly, The Last Guardian. I admit it, that was a very real factor, as I worship ICO. 4) other than TLG, the PS3 just had more exclusives that interested me. It's that simple.
The only thing I regret missing, other than some stray XLA titles, is Halo. I'd like to play that. Gears, however, doesn't interest me. Third-person cover shooters aren't my thing.
So, there you go, that's my rationale for going with a PS3. I'm sure I would have been perfectly happy with a 360, but then again, I'm perfectly happy with my PS3. I do think, though, that the PS3 is the more logical choice if you're also a PC gamer.
Actually, on second thought, I might have been a little biased against the 360. Nothing, and I mean NOTHING in gaming has made me rage like Games for Windows Live. That piece of processed hell came close to ruining Fallout 3 for me, which is unforgivable. To this day, I still haven't played my copy of BioShock, simply because it's also GFWL. Grr.