Playstation has some excellent exclusives, most of which have already been stated here. But one that I think is often overlooked, and one that you might not be playing because you said that online gaming on the PSN wasn't really your goal, would be MAG.
If you enjoy a shooter that rewards you for your efforts and is focused on teamwork and large-scope gameplay, then you'll really like MAG. It's not as easy to pick-up-and-play as a game like Call of Duty, but it makes up for that by being challenging, and by translating a single-player experience into a huge multi-player arena.
Instead of playing a single-player with hundreds of AI opponents, all of those opponents are real people, and they react very differently than the AI. You can trick them, sometimes you can hide from them, and you can sneak up. There's tons of possibilities that I could go on all day about.
Playstation just offers some very unique experiences, and they keep on attracting pretty awesome developers. Sucker Punch, Naughty Dog, Zipper Interactive, Guerrilla Games, Media Molecule, Atlus, Insomniac, Team Ico, Quantic Dream... All amazing developers in the Playstation stable, and that doesn't even count Sony's in-house stuff like God of War.
To just hold on to the XBox out of fan-dom is to miss out on some truly awesome gaming experiences out of ignorance. I bought my 360 for Halo, Mass Effect, and Gears, and that's about it. I really wish Microsoft would put some effort into keeping their exclusive studios, instead of just chasing them away or shutting them down. (Think about Ensemble - They got chased to Activision, of all places!)
So there you go - My reasoning for grabbing hold to Playstation's rising star.