Excellent rifle by the way...if I was going to go with a gas sniper, the M700 is certainly on my list. That said, if green gas is delivering the FPS and is readily available, why not stick with it? There's lubricants in green gas that will keep your gun working properly and require very little maintenance, whereas CO2 will dry out seals and such requiring a little more work.
There's no denying the punch of CO2, but if you don't really need it, why do it? For the same reason I love my 1911, I guess...it kicks like a mule!