I think if you want to try to futureproof yourself (if there is such a thing), the i7 is the way to go. In the future you will see more software take advantage of 4 cores where as today, most games/software take advantage of two.
The i5 is able to run at higher clock speeds in most cases for single threaded apps. So if you are using older software you will probably see it very close if not outperform an i7. However most software developed today is starting to take advantage of multi-threading. The more threads, the more you will see the performance gain of the i7
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