Although the intial speed or a 720 is slow, it has 4 cores, with 8 threads, which is huge, and it has that "turbo" thing, that will overclock it to 2.8ghz, which is huge, thats a 1.2ghz speed increase, almost double, I would go i7, no use in getting older tech, although it is good tech. it would be a better choice to go with a new chip, same goes for the 820.
820- 1.73ghz reg speed, 3.06ghz turbo :O