Chances are, at long ranges, one will be much better than the other. 2.4ghz if you have some obstructions. 5ghz if you have lots of height on one or both sides (it’s less tolerant of line of sight and ground obstructions but more gain for the same size)

At least wireless ISPs don’t bother with spectral diversity on consumer fixed links.

Predicting constructive/destructive interference… you’ll need to do some radio-mobile plots (I think that was the software with topographic maps built in).