I'm strongly considering the RT-N66U, but it's 11 x 14 footprint is quite large for the shelf I have devoted to my network equipment (and it's not simple, because all the 10Base-T is pulled to that spot). Is it possible to mount this thing vertically?
Not sure about the mounting question, but I think you have the size wrong. The 11x14 sounds like the size of the box in which it comes, not the router itself.
N66U footprint is 8.1" x 5.86" x 1.38" (WxDxH). Including the antennas, the height is 6.5".
I've had my router vertical, using the stand, since I bought it, it runs cooler that way. And it takes up a lot less space. Yes, you do have to play with the antennas, which isn't too hard using a laptop with inSSIDer or an Android phone with a network analyzer app. I'd be surprised if you have the antennas positioned optimally for both router orientations, vertical and horizontal, and saw a difference in signal strength due to router orientation.