I'm not sure about green gas vs co2, but I think the biggest difference in blowback vs non-blowback is the extra gas used to cycle the slide. I have a M&P replica, non-blowback, co2 powered. I like it and I get between 150 and maybe 200 shots per co2 cylinder. I also got 2 extra mags so I could practice rapid reloads. The only difference in reloading to the real thing is the cycling the slide. I wish I could get a blowback model for the extra realism in reloads, but I can live with just running my hand over the slide like I'm going to release it. That works for now. I'm not sure about what manufacturer is the best. I started by googling air soft and looking at different websites till I found the M&P witch was fairly difficult to find. Glock would probably be fairly easyer by comparison. Wish you luck at finding the right equipment for you.