You can probably do 14 gauge unless you have long runs to the speakers. 12 gauge is a little more costly, and might be a tad harder to work with, but wouldn't have any other downside. Anything thicker might not fit your speaker posts without banana plugs.
THX cert wire... well, if you want the bragging rights of everything being THX certified, that's up to you. That's the only benefit, though. Certainly doubt it would improve the sound.
I personally think everything Monster makes is wildly overpriced (though certainly well-built) and I avoid it, no matter how much they certify it.
Don't believe everything on the Interwebz! A duck's quack DOES echo!