I bought the bottle mounted Sterimatic gun when I discovered I didn't have the three hands to hold the sheep, the gun, and the separate tube - connected bottle of the other type. I did put the bottle in my shirt top pocket but it was still awkward and not as slick and flexible as the bottle mounted gun.
mab, you don't have to worry about tenting the skin because the spring loaded needle only projects skin deep when it is depressed. In fact you cannot get your fingers close to the injection site because of the Sterimatic thingy that sterilizes the needle as it passes through it. It means you do have a spare hand to separate the fleece and ensure penetration. You'll love it.