Personally I'd spring for the baseline MP:
For:
The iMac (as good as it is) still uses laptop architecture (or at least it did up until this revision) so they're are obvious speed benefits there.
The processors are faster, and can be pushed harder due to the superior cooling
The main benefit of having a MP is it's expandability. The thing could potentially last you 7-10 years! You can add RAM, new graphics card, new processor (not hard at all really) and you'll still be able to keep up with new tech (USB3 and Lightpeak due to the ability to put those cards in the pci-e slots)
Multiple Hard drives (Good for backups/RAID setups. For Photoshop it's a good idea to have you OS and applications on one HD and the images you're working on in another, as this gives you a performance boot and you have less to lose if the OS drive fails). It's pretty common to get a small SSD (60 gigs or so) put Snow Leopard and PS on there and use a fast mechanical HD as a "Scratch".
Against:
Power consumption will be higher with the MP (I'm not sure of figures but if you want to but they won't be *That* much, so if you're having to watch the pennies then it might be a factor
No screen: Obviously this is an additional shell out (I'm an audio guy so a nice vibrant, in your face colour monitor would be no good to you, however there's a thread on here about the best monitor to use with your MP for pro visual work)
You mentioned you don't like the iMac screen, so the Apple one is probably not the way to go for you.
I've run out of ideas for not getting a Mac Pro now

......sooo errrrr the MP takes up more space!
Seriously all in all I'd spring for the extra £300 (maybe budget £500 or so to add in some more RAM and a SSD).
Do you intend to make a living from it or are you a student/hobbyist?