I bought a 360 because it was just the new console at the time, lol. I was pretty keen for the Fable series at the time but the only real exclusive that I was invested in was Mass Effect after it released and we know what eventually happened to the 'exclusivity' of that. Plus, it was also good for backwards compatibility insofar as that every Xbox game that I was interested in was playable on the 360. Played through both KOTORs and Dreamfall. Oh, Morrowind too! I didn't get very far in that though. The games I had already owned on my original Xbox that I had since given away (Jade Empire, Fable and a few others) were all compatible too.
Then there's the added 'bonus' that Bethesda games are much better optimised for the 360. The state that Skyrim was released in for the PS3 was atrocious.
I had a pretty hefty PS2 library at the time but the PS3's price point, later release and then following that, its handling of Backwards Comp after it got cheaper made me less than eager to jump on it. While I use my PS3 more often now, I still prefer the 360's UI. It's loads better than the XMB in terms of use, clarity and store integration. I despise the PS Store on the PS3. Honestly, I would still probably be firmly on the MS train if not for the whole mess last year.
Hopefully you get the chance to play P5 though! You can't miss out on that!