This should really go into the Airsoft forum as neither of these are air rifles (they fire 6mm bb's, not .22cal or .177 cal lead pellets or metal bb's).
Having said that, I honestly wouldn'ty buy either. They both break in short order. Being from Canada, we don't have the same free access you guys do, so we have to be particular about what we do get. I'd recommend a Tokyo Marui myself. Fairly inexpensive and will last for years in stock form. TM's only real drawback is the plastic body which can break at the stock end. A metal body upgrade will solve that problem in short order. If you can get them there either Jing Gong (JG ) or A&K make some decently priced stuff, though quality is hit and miss. Classic Army make full metal guns, their quality has come a long way int he last few years, but are a bit more then TM.
I know you said fairly inexpensive, but if you want identical feel of a real M4, your only choice is a SysyemA PTW (Professional Training Weapon system). These are designed to mirror every aspect of the armalight series from dimensions to weight to accessories to even the size of the magazines. Unfortunately, PTW's sell for about $1500, less battery and mags.