I would have to say speed, but I think 280 fps is plenty fast enough with my 403 grain arrow. I like a balace of speed and KE. If you look at the numbers, heavier arrows don't providemuch more KE. It is usually less than 5% so becareful how much you trade off with those heavy arrows.

here is a good calculator to determine KE, arrow drop and momentum

http://home.att.net/%7Esajackson/ballistics.html