The steel barrel nut ($14.99) can be torqued down to a higher value than the aluminum ($19.99) 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 (3.29 ounces) version is heavier than the aluminum (1.4 ounces). If you are going for a super lightweight build, then aluminum is your best bet.