I'd go for the i7. Man has to think of the future: newer versions of software, more megapixels in images, etc. Besides, only one application running is hardly the case, usually one has a number of them running at the same time.
FWIW, regarding RAM and someone mentioning 8GB not being enough. My i7 system has 16GB and I managed to hog it up with PS and LR once. I had LR 3 running and about 10 hi-res TIFFs with several layers some of them open at the same time. Memory consumption went into the 90%'s and the computer was unresponsive for several minutes. It took easily 10 min or so for PS to respond to my clicking the X and closing.
When it comes to CPU power and RAM, the more the merrier.