It would help to know more about your needs ( like is 3D a requirement?), for us to offer more solid advice.
That said, I can speak to the MX-120s abilities. You probably know that the HDMI inputs are video only, so you'll be running audio via coax or toslink.
not a huge deal unless you require the latests and greatest Dolby processing in your ...processor. That's about it for the downsides (for me).
The MX-120 offers excellent 2-channel and surround sound and is extremely flexible with regard to inputs and their customization. They're also somewhat of a bargain in the McIntosh line since they're shunned by the 2channel purists, and the home theater guys who need to have the latest decoding capabilities. It's perfect for my particular needs. I wanted a great 2-channel processor with the convienience and ease of use for my wife's TV and movie needs, without having to buy another 2-channel preamp with pass through.
I don't know much about the Rotel, but I imagine it would need to be shooting pretty high to match the sound quality of the MX.
Another consideration since buying used appears to be an option, is some of the offerings from Anthem. They get great reviews, but sound like they could be a bit of work to get set up properly.