You can mount it however you like, you just need to consider how signal propagation will be impacted. As you're comparing two cases where, in both, the antennae are vertical and the install position is about the same, I can't imagine there will be too much difference, though always worth doing a quick survey to double check.
The main concern really is that you might be mounting it close to something in appropriate. Try to keep transmitters away from anything big and metal as much as possible as it impacts all sorts of RF things which ultimately hurts performance (in the broadest sense of the word).
