What is the difference between a grip-nut and brass check valve?
Brass check valve is made of brass.
Grip nut check valve uses the plastic nuts to screw the tubing on for a tight seal. If you are describing the one I am thinking of, then it is made of plastic.
Plastic eventually hardens with time. Brass does not, but you have to get a quality check valve; I have tried a few and they end up not working (allow water to leak backwards), so I've given up and just stick with plastic check valves now (and replace them every so often).
For your questions regarding pressurized CO2, might I suggest you take a look at my guide to pressurized CO2 (linked in my signature below).