Good question, in theory the i7 might be a higher binned chip which *might* net you a higher overclock. But honestly probably not. Making CPU dies is sort of like making cookies, you know how some turn out just perfect in every way, and some, while still very tasty might not be exactly round?
I know its a silly analogy, but thats sort of how it works, at least thats my understanding of it anyway. The ones that turn out the best will go into the higher line, the ones that aren't quite as perfect will have features disabled (like HyperThreading in i7s), or they'll lock the multiplier and use it as a non-K i5, or cores disabled and be used for i3s, etc.
But even so, since the i5-3570K is meant to be overclocked, its probably going to be one of the best. So no, I don't think an i7 is really going to have any better potential to OC. But even so I'm more of a conservative overclocker, I wouldn't push a CPU as far as it can go to the red line for a daily OC. 4.5GHZ is a hell of an overclock for a 3570K, and if you made it there, I'd leave it be. Keep in mind, you're only guaranteed to get stock speeds. Individual results will vary from chip to chip.
And no, I don't think one is easier than the other to OC.