Welcome to AC.
Your choice depends on your speakers and goals. If you have high end or revealing speakers then go with the MRX1120, you will hear the difference.
If you plan to keep adding speakers up to a 7.2.4 Dolby Atmos then the MRX1120 has you covered. The MRX720 also does Atmos but you will have to bring your own 4 channel amp, which is not necessarily a bad thing but it does add more boxes and complicates life for people who are not electronically inclined.
I have an Anthem AVM60 which also has the toroidal transformer so I have a stack of amps out of necessity. I picked up a used Adcom GFA 7300 5-channel amp for the Atmos channels, 60 high current watts per channel which is more than enough. The home theater also has more headroom with separate amps for those important immersive events.
