There will likely be a fairly noticeable difference in performance. The more expensive one will of course be faster with visual work, but the real question is... how much are you willing to pay? What you're willing to pay will ultimately determine which one is best for you.
Go to the configurator, choose the lesser 24" iMac, and build up your options from there. (Don't opt for the RAM upgrade though-you're better off buying 4GB separately from Newegg and installing it yourself, and selling the 2GB the iMac came with). As a baseline with Aperture, I would expect you to be happier with the upgraded model, though you might not need 500GB of storage. I suggest:
2GB RAM (upgrade through Newegg.com)
nVidia GeForce 8800 GS
320GB HD
CPU - up to you
Aperture pre-installed
Total~$2200 with 2.8GHz CPU
