The specs for the 2 are the same for uhf.
CM7775 UHF: 26db gain, 2 db noise
CM7777 UHF/VHF UHF: 26db gain, 2.0 db noise; VHF 23 db gain, 2.8 db noise
The only reason to purchase the cm7777 is if you were to add vhf.
Why dont you add another 91xg installed side by side like these guys did.
http://www.atechfabrication.com/products/dual_antenna_boom.htm
http://www.bobmerritt.com/dtv/dtv.htm
another 91xg $90
dual antenna assembly $110
center boom bracket cb1 $40
you should also purchase the balum harness $35
http://www.atechfabrication.com/products/phasing_harnesses.htm
use rg 11 from the antenna to grounging block or to fist split. you can order the rg11 to length with fittings
http://www.atechfabrication.com/products/coaxial_cable.htm
Add a rotor
and amplify with your cm7775.
looks like a fun project and should help.
also play with height at antenna web and see how it affects the results. you may also need to go higher.
I know money does not grow on trees, but adding the antenna with the other stuff may be the answer.