Definately the M9. I love my G18, but I go for gas guns first as they tend to be more reliable and simply perform better than AEPs. True the G18 is full auto but full auto pistols are over-hyped to say the least. I have three 80 round clips for my G18 and I was just getting tired of having to reload all of them for a gun that has such low fps in the first place. The G18 was accurate on full auto but it has such a high rate of fire its over in an instant unless you spend more money on hicaps which would kill your price range anyway. Thats part of the reason why I got a G19 gbb instead of the G18C (the G19 is semi auto only). I have never used C02 but I do know this: You wont have to refill the resivoir after every clip like with the green gas M9, and for that price the power you get is astounding. HOWEVER having said all that I must stress two things and I hope you listen: 1)dont buy from SpeedyToys as I and others I know have had numerous bad experiences doing buisness with them and 2) Dont get that M9 and go for the Y&P M9 NBB. Its green gas but you wont really be missing out on that much power since the Y&P still puts out about 320-330, its just a couple dollars more, its way better build quality and I know first hand thats its accurate at close range and ranges outside 75 feet as well, its more realistic and fun to use because it has a full sized clip and you wont have to open the grip up, having one gas can is arguably easier to manage than a bulging pocket of crosman Co2 cartridges, and Y&P is one of the best brands out there for NBB pistols. I have used it and I loved it. Even this would still be a better choice than the G18.