I do think vertical feed is somewhat faster, but the good thing about power feeds is that it is harder for balls to bounce back out of the chamber. This doesn't apply if you have a diagonal feed that isn't a power feed (direct feed).
What sucks bigtime about direct and power feeds, is that they offset your hopper to one direction. This is particularly bad in right hand feeds, as most people are righthanded. I'll explain why in a moment. Also, because the hopper is off centre, the marker becomes somewhat less balanced.
Now, the reason you don't want you hopper sticking out to the right (when you're righthanded) is because it will be sticking out from behind your bunker than with a vert feed.This means you'll be a bigger target. Bad!!! Can you tell I like vert feeds?
Some of this is fact, some of this is personal preference, but I hope it helped or explained things somewhat.
Okay, this last part I added by using the edit option. Sorry to be telling you what someone else already said, but it looks like Onion and I posted more or less at the same time, cause his post wasn't there when I started typing. Good looking out Onion!