Leviathan by Scott Westerfeld
Very briefly today as I'm racing towards the end of NaNo, although I doubt I shall make it!
The UK one is an illustration from the book, so it is a little more representative. It's interesting how both covers seem Clankerish rather than Darwinist (if you haven't read the book, check out my review for what I mean by that statement).
I like the UK best. It gives me more of an idea of what the book will be about and I love the curve in the title. Although it's a very close thing. I LOVE them both.