Both are masterpieces - what a horrible choice!
In the end, I went for "Master..." simply because it's a more rounded album and covers a greater divergence of styles. Every time I hear "Battery", it just floors me with its rampant energy, and "Master..." itself is near-perfection. I'm not so keen on "The Thing...", but "Sanitarium" rounds off the best side to the best metal album ever... oh - this isn't supposed to be a review
Reign in Blood is a masterpiece because of several things - the most striking being that it's a full 10-track album, all songs have their full complement of verses, choruses, bridges, solos, etc - and yet it's all over in 26 minutes.
The unique out-and-out aggression of the thing makes it a spellbinding and breathless experience - and also an exhausting one.
Reign in Blood, like Master of Puppets, is a landmark in metal - something that had never been done before, in a style that is almost completely unique to the band, and something that can never be repeated.
South of Heaven didn't try - it dropped a gear or two and concentrated on getting a darker, heavier sound, like Black Sabbath on kettamine and qualuudes. It's still a great album, but not the masterpiece that "Reign in Blood" was, or the benchmark that it remains.
Hmm.
I can't seem to disengage review mode today...