I ended up going with Coverdale, but I love both, and listen to both regularly.
Bach was crazy good on the first two albums, especially Slave to the Grind, but he hasn't done a single song since that I've really enjoyed, and his voice hasn't impressed me as much since. Coverdale has had a much more consistent output over a long period, and he has such a distinctive voice and style, and even at 60+ his lower register tone is amazing.
Also he's just the classiest dude in rock.