The fastest one with the most cores that you can afford, and as many of them as you can afford, supported by as much RAM and disk space/performance as you can afford. And you'll want a good video card to go with that.
With an unlimited budget you could get a new Dell Precision T5600 with dual Xeon E5-2687W processors and it would likely kick the butt of any other computer on the planet right now running Photoshop.
I like looking over the benchmark results here:
Thanks Noel - we'd all love to be able to have the best but I should have mentioned that sadly my budget won't allow that sort of 'shell out'!
I don't work on huge files but need PS to run smoothly and quickly especially when making adjustments in camera raw.
I'm having a system built to order and have been considering an i5 2500K Quad Core or the Ivy Bridge i5 3550.
Not sure which would suit PS best.
Either of the processors you mentioned will do a fine job. The very latest generation tend to get more work done more quickly in a given core, so likely the 3550 is likely to be a little better. Higher numbers on the benchmark site above are better, and I've come to think that you really do get what you pay for with microprocessors. There's almost a perfect parity between performance and price.
But if you have to trade financially for more RAM vs. more processor speed, opt for the RAM. I strongly recommend shelling out the additional $$$ for ECC RAM.
And as I mentioned, the video card will have a lot to do with helping you achieve fluid operation as well. Shoot for something that rates 2000+ for "future proofness" on the video card benchmark site I mentioned above.