i5 and i7 are the EXACT same (of most models, not counting Sandy-Bridge E) except for two things. 1) Hypethreading (HT): This allows two threads to run on the same core, giving you 4 "physical" cores and 4 "virtual" ones, which the computer treats as a total of 8 cores. Now, don't confuse this as having 8 core perf. as a thread is not a core, essentially you'll see that clock-for-clock and cache-to-cache, in heavily threaded apps (most ppl don't use these) a 25-40% increase in performance. 2) Cache, not a as big a deal(still important!), but i7's tend to have more. Granted though, most of the intel processors in iMacs, (not counting BTO) are S versions or low end versions.
tl;dr unless you do heave processing you won't notice a diff.