I prefer right side feeds nearly every time even regardless if the gun is pump, mech-semi, or even electric-semi.
The reason why is because of my other hobbies. I am more into firearms than paintball (especially sub-machineguns) I want to hold and aim my paintball guns in a similar manner to that of a firearm with a stock and a big part of that is having the least unobstructed sighting view as possible. I like aiming down the center-line down the barrel and with a vertical feed I either sight down the side (Bren style) or tilt the gun and I don't like to make a habit of this.
With that being said I own a few vertical feed guns and like them but would prefer they came in a right feed version.