That’s a tricky question and I feel like the answer is very personal. Simplifying it too much:
Organic Maps: basic, simple, easy to set up and understand.
OsmAnd: heavy, complicated and with looots of information.
I personally find that Organic Maps works for 90% of my needs: it’s easy to use and navigate through. The remaining 10% I need more advanced functions not present in Organic Maps and OsmAnd comes to the rescue.