US // UK
I love the UK cover for The Monstrumologist -- It's just so fun and weird. Plus, there are blood and guts present, which is always good for a horror story. I do like the US one, just not as much. It seems to be aimed at older readers, while the UK one looks like it's aimed at a younger audience. Is it just me that gets that impression?
I'd buy either, though I'm voting UK this week. Also note the difference in title... I wonder what prompted that?