Since I read the word "game" (even light gaming today - possible heavy duty gaming in the future), I'd go with the 21.5" i5 processor. Especially if you like keeping your computers for 3+ years. And, knowing that future re-visions of apps are taking advantage of multi-thread Ix processors.
On a side note, don't think I ever read a post from a person buying a computer with "too stong" of CPU in it and regretting it. I ready lots of "shot - I should have bought faster, better, stronger CPU" type posts. But, don't think I ever seen posts from folks regretting their "too much" CPU. Especially for long term usage.
Perhaps one day, Apple will design a machine that allows someone to simply remove and replace the system's CPU chip. Vision another flip down door. Remove its "slow" CPU and "slip in" a better CPU. Just like adding more memory. That design would be really cool... Perhaps in time...
Hope this helps...
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