I have an XR18, which is really the same mixer as far as features go. I relegated it to backup duty when I got a Soundcraft Ui24R. I've never heard the MR18 but the Soundcraft sounds noticeably better than that XR18. Let's assume the MR18 and Ui24R have comparable sound quality. The Ui24R is a far and away a better mixer. For ~$100 more the extra capability really makes it a no-brainer.
- 4 more XLR inputs
- 2 more aux sends
- discrete track recording straight to thumb drive plus 2 track master
- web based mixing (no special app or software needed)
- subgroups that can also be routed as matrix
- feedback suppression on all outputs (works pretty well)
- usb music player
- ability to directly connect a touch screen (or regular screen and mouse) right to the mixer. This gives you a last ditch, but completely reliable interface if wifi just goes bonkers.
- De-esser on all channels
- 2.4 and 5 Ghz router (only can do one or the other though). It's significantly more reliable than the XR18 router, which I believe is the same one in the MR18. It's still not awesome, but OK for small venues
The only thing you're giving up is the large list of "plug ins" that the Behringer... um I mean Midas, mixer has. Other than the multi-band compressor, I never really used anything that the Soundcraft doesn't have.