I have both the 75 and the 45 both in .177.
They are very similar in weight and balance, the main difference being recoil and power.
The 75 (pnumatic) shoots sweet as a nut totally recoilless at about 3ft lbs and very accurate in the right hands
The 45 (spring/piston) is the powerful one very close on the limit, still as accurate with plenty practice due to the recoil.
I would say if you are able, buy both lol.... basically if its for paper punching buy the 75, if its for general plinking and some paper punching buy the 45.
Air Arms S400 Nikko Gameking 4-16x50....