I use Photoshop and Painter on my 17" Dell Precision Mobile WorkStation M90. Lately I've been getting some shoulder numbness from lugging it around. My co-worker has a 17" MBP and he loves it. Its much lighter then my laptop and he doesn't seem to have any issues with it. I thought about getting the 17" but I would like something smaller lighter. I'm not sure if I should get the 15" or the 13". If I got the 13", I would use a monitor for it. But I'm a bit worried about processing speed with the software I use. What do you guys suggest, is it the 13" or 15"?