If you stick with the MonoGame APIs, there is little to no difference between the two.
@KonajuGames OK, that is good to know.
Yes, single code base cross-platform. Where "cross-platform" means Windows + iOS + Android.
Currently I have a relatively small amount of DirectX code; most graphics code is XNA.
But fairly soon, will be writing more 3D code; likely some of that new code will be low-level.
If I was ever going to switch, now would be the best time.
The catch is that my company doesn't need this code on iOS + Android any time soon, but it does need new 3D code. So there is a short-term vs long-term trade-off here.
Based on your reassurance that the platforms are in sync (I'm unlikely to do anything so cutting edge that it hits any rare differences), it is worth taking a couple of days next month to try OpenGL versions of what my company did in DX. If that goes smoothly, then I will likely continue with OpenGL. (I have previously done some OpenGL ES 2.0 coding, so I wouldn't be starting from zero knowledge.)