The 91XG is very strong for UHF above ch45, BUT WMBC will move to ch 18 with the analog shutoff in 2009 and the 91XG is very weak by comparison in the lower UHF range (~4dB lower than PR-8800).
I do not have raw gain data for the HD8200p but the Winegard PR-8800 has a fairly flat gain across the spectrum of UHF channels.