I'd personally go with the Canon, but I'm a Canon fan.
The 60D is a little higher megapixel which barely matters anyway and it's only 1 or 2 higher. It's also weather sealed, whereas the D5100 is not. That to me, is a big deal, if you plan on doing outside photography. I'd never buy a camera that isnt weather sealed but I want to be able to use mine outside in slight rain without worrying. The 60D also shoots 1 more frame per second than the D5100, not a big deal, but if you do wildlife you might appreciate it.
The Nikon is a smaller camera, the Canon is slightly larger and heavier. You'd need to hold them both in your hands to see which you prefer. I like the size of the mid range Canon cameras like the 60D better than the smaller/lighter ones.
And finally, the 60D will control a flash remotely, which if you really get into tank photography, will be great. My brother has a 60D and uses the remote flash control a LOT more than he thought he ever would.
Anyway, that's my 2 cents! As far as lenses go, you'd be happy with either.