I just upgraded to the new 15" MBP from the same model you currently have. I am so happy with it. The footprint is actually slightly larger, and I think it's barely thinner. I can't tell the difference in the weights. But it's all about those curved edges. The overall feel is fantastically slim.
Everything is nicely faster. I have a 60GB iPhoto library, with 50k photos. Scrolling through them vertically, or horizontally (in an event) is so smooth. And Faces and Places are several times faster to access. Also much better lighting adjustments with the iSight.
Battery life is pretty impressive. With hefty synching to my Atv and Time Capsule, I'm still getting 4 hours easy.
I play WoW and my early '08 was so hot, I had SMC running full out plus a fan base underneath. This one, I can just keep on my lap. No more scorched thighs, hurray! The whole feel is more lap friendly, including the mag plug.
It's not a MBA, but the difference just got a whole lot less. Adding the spec difference, I wouldn't consider the Air anymore.
Just to seem a bit more fair and balanced, let's look for negatives. I'm not a fan of the glossy screen. In every environment I'm in, the glare or reflection is so noticeable and usually very distracting. I'd have to say it knocks a couple points off the experience. 2 thumbs down on that one. Also pretty neutral on the buttonless trackpad. Still adjusting and not seeing an upside. It's hinged at the top so you're still aiming for clicking towards the bottom. Not what I expected and not 'clickable anywhere'. Softer touch to the keyboard though, which I like. Oh, and the superdrive is as loud as ever.
Overall a solid B+. The glossy screen really hurt.