I assume you are running Photoshop CS4-Extended, correct? In terms of raw CPU power, the i7 will be the strongest, paired with that 6gigs of ram. I would go for the 720QM personally. I feel your pain, working on 1.5+ GB files was a huge pain in Photoshop CS3 with my old Athalon 500+ x2 and 2GB of RAM. :P
What OS are you going to use this for? And which version of PS?
I've always wondered about that given that the 720 is 1.6 vs 2.4 for the 520. Does Photoshop really make that effecient use of the 4 cores that it would make up for the difference on operations that are restricted to one or two threads?