If you get an SSD for the Pro, it will outperform the Air. If not, they'll be about the same (I have a 17" MBP and a 13" MBA). With image processing, disk speed is important, and the SSD gives a big boost.

Don't worry much about the RAM - OS X manages it much better than you're used to with Windows. I've got 8 GB in my MBP, and the only time I come anywhere close to needing it all is when I'm running a Windows virtual machine and I have to give it 5 GB to keep it happy.

I can tell you that my Air handles large image files in CS5 very well (not just 5DII files, but much larger 16-bit files from scientific instruments). I've been able to slow it down slightly with 2 GB images (slide-scanning microscopy files), but it still does ok.

The Air is absolutely great for travel. But, unless you'll be traveling a lot, I'd get the Pro - mainly for the FW800 port. Sure, there are a couple of Thunderbolt hard drives, but they're uber expensive and likely to be that way for a while. I'd go crazy if I had to move all my images over USB2.