I also liked the fact that the Yamaha's center is actually a center rather than just putting another Behringer in front since I use this for my computer/media center.
Vertically arrayed is best. The best choice of center speaker is to use the same speaker as the mains. Best placement would be identical height. You are then given the best horizontal dispersion, without lobing, the best sonic match, and the even height of drivers allow perfect panning.
OTOH, some of the things you may give up with a smaller bookshelf center is output capability and extension.
There is a reason why the best theaters in the world strive to have three identical speakers in the front as vertically arrayed (almost uniquely allowed by acoustically transparent screens).
I'm sure you will be pleased anyhow, with your choice, but I wanted to educate you on a certain matter. Something I wish was done for me earlier in my hobbyist experiences, though I finally did learn.