While both bands had phenomenal output in the 80's (and similarly went commercial, and then declined drastically in the 90's and afterword), for me, this is an easy choice: Queensryche. The sheer brilliance of Operation Mindcrime alone, my favourite album of the decade, goes far beyond anything Metallica accomplished IMO and makes this makes this an easy win for the 'Ryche in my mind. Add to that the pure excellence of Rage For Order, The Warning, and the debut EP, and Queensryche's victory is further cemented for me. Queensryche was so far ahead of the curve musically at the time, and their music was light-years beyond the edge of what any other band was doing in the period. 80's Metallica, while awesome it's own right, was really just an American version of NWOBHM (especially Motorhead and Daimondhead) on steroids. QR is one of very few bands in the 80's that was truly unique, and not simply a re-tred or reinvention of something else, and the albums they put out in the 80's were records that no one else could have done.