Truth be told, unless you're whittling away in your bathroom, shaking all over, ready to sell a console or a liver to get your next fix, I'd say keep both. Thing with consoles and games as well is that as soon as it's in a consumer's hands, the price drops like a mother fucker. If you sell your 360, don't expect any significant amount of money, and if you sell your PS3, then it's as if you tossed around 200 bucks out the window.
To me, seems like a choice between getting (sooner or later) what you pay for, or wasting a nice couple of greenbacks.
P.S. On a personal note, I've had a 360 since day one, and just recently saw incentive to get a PS3. I must say, I'm damned glad I did. MGS4 (not as perfect as people say it was, but a very good game nonetheless), Heavenly Sword, Uncharted are all experiences that I'm glad I didn't have to miss as a gamer, and now I can look forward to both the likes of Fable and Gears, as well as LittleBigPlanet and others. And yes, Blu-Ray is all that and a bag of chips.