Which one of the two is the best buy (Most Bang 4 the BUCK) Emotiva –XPA 11 vs Monolith 11x Amp?
I'm definitely not a pro adviser when it comes to the Amps, but consider that an 11 channel amp is possibly not the best decision.
I know for Emotiva, the "Stereo Modules" offer much lower power output than their "Mono-Blades" and they DO NOT discuss what their output changes to when you have all 7 modules working. For the Mono-Blades, with 7 modules, they will output 200wRMS to the 3-channels supported by those units... (you will likely see more power in real world usage because the All-Channel-Driven RMS thing is not a very accurate guide, but... ) so speculating what happens to the output of those stereo modules, I would expect maybe 40wpc RMS, ACD (?). That just doesn't seem to fulfill the bang for the buck target.
The Monolith Amp on the other hand gives you a clearer expectation of how it operates with 200w ea. for the Front 3, and 100w ea for the remaining 8 channels.
Ultimately, the question should be considered from this point of view:
What is the goal for your system and listening experience?
Granted you likely don't need 300 wpc for Atmos, or Rears. Ever.

Unless you have horribly low-sensitivity speakers... but I digress.
Are you building for full 11-channels now, or in the future? Do you have the speakers you want to use in mind?
Do you know what power your mains require for listening at the SPL you want at your LP?
Peak SPL Calculator is a great tool to help you see what power you might need for your speakers.
I would recommend considering a good AVR that can power your Atmos and Rears (and maybe your surrounds), putting only your Front 3 (or Front 3 + Surrounds) on external amps. To me that makes more sense.
Hope this helps.