RAM will be the better option for 99% of users.
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You will see minor differences in benchmarking applications and CPU intensive tasks will be slightly faster, but in a practical sense, you won't see notice this. If you really have CPU intensive tasks, then the quad core 15" or 27" iMac is a much better choice.
The only other difference I can see is the Graphic frequency or something, the 2.6Ghz is slightly faster than the 2.4Ghz and its on par with the i7