The vertical feeds are designed to be able to feed the balls into the markers faster. This is a good thing to have on high end electro markers that are capable of firing off 13+ balls a second.
A power feed is more or less an side feed, but instead of the balls going straight into the breach, the feed is attached sort on top of the marker, which makes the ball drop through a 90 degree angle, which prevents the paintballs from bouncing back up into your hopper when they get hit by a blast of gas when you fire your marker. The automags really benefited from this, since they fired fast (for their time) and they use a pressure way higher than today's top end markers. This often caused the balls to jump up into the feed tube, meaning that when you fired again, there was no ball, or worse, it was only half way in, so it would get chopped. The powerfeed prevents this, because when the ball jumps up, it just hits the side of the feed tube...
As far as aiming is concerned, it's not a problem with a vert feed. You can look down the side of your marker, and even better, you can look 'through' your marker. Despite popular belief, aiming should be done with both eyes open. That way you can see the distance better, plus you can look through your marker. Give it a try, you'll get better results with a bit of practice.