Screen is different its not as colour accurate.
Really? Everything I've seen says it's the same panel?
Quad cores are too slow for me - so unless Apple do the 6 core Intel this year, its useless for the money - I need at least a 6/8 core - ideally 8 for LR/photoshop/video (its faster not just because of what the apps doing, it keeps the rest of the system fast).
No ECC ram either
Nowhere near as good GPU on iMac - but its good enough for work stuff, photo, video etc.
I'm sure you know your own requirements better than anyone else, so if you'll notice a difference in the Pro then go for it.
Like I said earlier, I do design and photo work on a daily basis as well as some video. While I'm sure the Pro will be faster, I'm not convinced it will be so much faster that it would justify the additional expense.