I find that deciding on number/types of powerheads depends on the tank dimensions and it's inhabitants. For example, in my 75g (back when it was a reef), I had 2 250gph powerheads and 2 175gph powerheads because I found it easier to get the flow patterns to my liking with more powerheads that were less powerful. Now that it is a FOWLER, I have just the two 250gph powerheads (not looking for as much current in my FOWLR). In my 44g reef, because it is a pentagon tank, the current seems to swirl around the tank and end up making very high flow areas. For that reason, I'm experimenting with fewer stonger powerheads and more weaker powerheads, but I haven't found a setup I like.
Ultimately I don't think it matters... Just make sure you have decent flow through the entire tank, and that you have spots of high/medium/low flow as necessary to keep your corals happy.
