The only downside I can think of would be if you "abused" the larger window. The SRO will be a lot more forgiving of your presentation so even if it isn't perfect you'll still see the dot where the RMR could result in the dot outside the window. A sloppy draw when you have to pull your carry gun would be a bad time to learn that you've developed a bad habit.
If you plan to compete with the training gun also then the SRO may be the better option as you can always practice with the RMR occasionally or with dry fire.
If you aren't competing, will only be using it for training, and don't intend to move the carry gun to an SRO down the line I'd probably go with another RMR.
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