Maybe a dumb question, but I am curious..
It's a matter of personal preference, how you like to ski, and what kind of skiing you're doing. I'm sure someone is going to say, "short skis suck, long skis truck" - and there's some truth to that... if you want more speed and more stability at higher speeds, go longer. And of course with powder skis a little extra length will give you more surface area and assist your flotation.
There's really only one answer to your question, and that's another question: for what?