Lorefin

I wanted to buy a replica of Steyr Aug by GFC Guns. The GFG29 A1 and GFG31 Para has the same price. I assume the gfg31 version is newer and somewhat better but it's version with a smaller innerbarrel length and gfg29 has a longer barrel and im mostly shooting in the forst. I know it's a changeable part but well it's an aditional cost.. Was anyone of you using these guns and can tell me the difference?