First of all, you can't make sure, and the chances are pretty low you will get one, the majority of iPad 2,4's are not refurbished, but are the 16gb that apple still sells as new, so if you plan on getting a 32 or 64, you won't get one with the new chip.
I personally own 2 of them, and there isn't much difference at all. Slight, slight performance improvements in very CPU heavy processes, only noticeable if you actually time it.