If you're looking to get stronger, just race every opportunity you have. No sense in targeting only road races. If you've been racing for a while and know your strengths and weaknesses, then I could see targeting only RRs.
The first year I raced I loved crits and didn't really care for road races. The second year I loved road races. Now I like both pretty equally, although I seem to have better results in road races. So if you're just starting out you probably don't know what you like or are good at.