This is a classic debate. If you like park and want to ride a good park you need a park city pass, period. It's the best park in Utah hands down, and one of the best in the country when it's in its prime.
Pow at park city is not so great though, they're at a lower elevation so they get less snow than Alta/snowbird/brighton/etc and have some fun terrain, but not much. So for pow you have some choosing to do, and I'd chose mostly based on where you will have friends to ride with. Try to find a good group of friends before buying your passes, because this is probably the most important part of having a fun winter. Not only is it fun to ski with people, but having people to carpool with and whatnot makes it cheaper and more fun as well.
In my opinion, the best two pass set up is Park City and Alta.