900's would have better vegetation penetration as long as its not extremely dense, as higher frequency has better bandwidth but lower vegetation penetration and lower the frequency, lower bandwidth but better penetration....but they'd likely be older, removed-from-service radios.
I'd perform a site survey and see what the noise floor is, if not a lot of ambient 2.4 GHz I'd try that using a higher power/higher sensitivity radio like the Nanostation M2 which is rated 23 dB minimum. BTW, just saw them
in stock at Ubiquiti for $89 which will NOT last long if it is, in fact, in stock and at that price.