So the steel barrel nut is going to be stronger and can be torqued down to a higher value than the aluminum version. The steel will also last longer if you plan on torquing it down multiple times.
The torque value for the steel is: 30-80ft/lbs while the torque range for the aluminum is: 30-60 ft/lbs.
With that said, the steel version is heavier than the aluminum. If you are going for a super lightweight build, then aluminum is your best bet. However, if you don't mind the extra weight and want the strongest option we carry, then steel is the way to go.