I have both and use them for different things. For me one of the biggest advantages of REW over OM is that I can make one set of measurements with REW and then go back at any time and examine them in different ways. For example, I can do one sweep 15Hz to 20,000Hz and go back at any time later to look at just 15-300Hz, examine waterfalls, spectrograms etc etc or I can use no smoothing and then go back and use 1/6th smoothing with no further measurements. I find this is a huge benefit to me, because with OM I always seemed to find a need for a measurement I never took or never saved, right after putting all the gear away. I still like OM for its ease of setup and use though. Plug the mic into the laptop and I am measuring!
I agree and add that the use of OM demands that I handle the player remote in addition to the mouse/keyboard. I usually forget to stop the test signals when I conclude a measurement in my haste to see/process the results. Then, I get angry at the annoying noise. Any vexation with REW is internal.