The i5 mostly would come in handy for more advanced game emulation play. The i3 that I have here in my Den is more than adequate for video playback (especially with the new Xine that we built that does va-api acceleration.)
My NUC is mostly in an open area behind the TV, so overheating really isn't an issue, but, the device itself uses a mobile i3/i5 CPU, so a little less oomph, for a lot less thermal output.
-Thom