if it just came down to 18 versus 14.4 then someone explained it to me that the voltage is the power and the amphours is the tank, you dont need a nutter machine to fire drywall screws in, my little 14.4 site cordless drill works all day for me.... if you wanna be sds drilling concrete then yeh, go for the 18, 24 or even 36v but for drywalling 14.4 is easy adequate, what you wanna worry about is how far it will go on a battery, and 3ah is plenty, enough for tony even...
when it comes to screwguns though, i hate the f'ckers... wont use em, but then the most ive got to do is board 1 ceiling a day...
if you really need one though, id send the makita back, listen to the lads and buy the hilti... you will never go wrong with a hilti...