gdgt

It really depends on what you want to use the computer for. The higher pixel density of the Air, longer battery life, and slimmer form factor are definitely advantages. But if you maxed your current MBP out with 8GB of RAM, I get the feeling you need raw horsepower? The Air is a fantastic computer for everyday use, but the second you do something processor intensive (video encoding, gaming, etc.) you will definitely notice a slowdown. Definitely not a show stopper if you only need to do those once in awhile, but if you're doing those tasks every day I would throw an SSD in your MBP.

An 11" MB Air will cost you around $1200. For that you'll get an amazingly thin, and light laptop that will in most cases be very fast. The exceptions would be the CPU performance and of course the GPU. It obviously has advantages in terms of portability vs the larger MBP.

You can guy a 128GB SSD for your MBP for under $200. And you'll find your MBP to be generally quicker after this upgrade. I doubt it will be quite as quick as a new MB Air at things like sleep/resume or even booting. It also won't have a thunderbolt port. But it'll have a larger, more spacious screen, a better keyboard, and of course a much better GPU.

All depends what you want. Either approach is good.