Really? Declaring a category that's winning invalid because the ones you like are losing? So predictable.
The genres are very, very similar and there is plenty of overlap. Plus it eliminates the bickering about whether or not the book is a mystery or thriller or crime or suspense or whatever, all are welcome to be nominated.
Mostly I nominated the two together in order to make room for other categories to be chosen for 2013. I like broad categories, let the nominations sort out the books people want.