I may be mistaken...
But I believe that that's not quite true. Vertical feed, feeds into the top as Jamie said, but what you called power feed I think is just side feed (left or right). Power feed actually refers to a curved plug that fits into the feed tube. this curve is lubricated by the air from the previous shot allowing the balls to feed faster than otherwise allowed by a side feed setup.
As to which is better, I think that that has a lot to do with personal preference, but if you are playing speedball, a right hand feed (as most dide/power feed setups are) will emerge from the bunker first, meaning that unless you tilt the body as you shoot, the first thing that you present your oponent with is a hopper, not good. So given the choice I would go with a vertical feed, but that may have sighting issues for some people. That all said, cheaper markers appear to be side-feed, more expensive ones going for vertical feed, I don't know why that is.
I'll just see if I can find a picture of a power feed on this sight, I think there will be a few
Richard