personally
if you are running on Co2 then go for the Front reg mount option, impy's are very tall guns on their own so having a big ole max flo on the bottom is gonna be like having a double decker bus cleverly disguised as a paintball gun! If you are planning on moving to air later then some would say keep it bottomline because a maxflow 4.5k will replace it and you don't need a secondary reg. However! with two max-flo regs on the gun you can expect a consistency of around +/- 2 Fps. Buy some gloves though because your reg will completely frost over and i have actually stuck parts of me to it before which i can assure you was not a pleasant experience
also get a tank cover if you can to stop your arm sticking.
