I've got both alloy and steel, under even abnormal use both are fairly indestructible.
Weight wise I know which I'd rather carry.
As far as mag couplers, there are a slew of aftermarket ones out there and almost 100% that one of the usual suppliers of HK parts would have factory ones. Google "G3 magazine coupler" or "HK91 magazine coupler" and you'll find a few.