RF is easier to me for available darkness phuzzygraphy. As with your Retina, quieter, and depending on lens, ability to see out side the picture area. 35 & 50's tend to be the more popular focal lengths.
If you do get a modern RF, keep the OM and compare shooting with both bodies and same focal length lens.
If you buy used RF and buy it right, you can sell it on at no loss if it doesn't fit. The SLR has lost so much value already, you're effectively giving it away.