We have done both. I would see if you can ask around and see what league most kids in your area join. It's nice to be with friends.
The main difference I have found is that right now with rec soccer, as far as I understand: BAC plays with teams of 2 calendar years and BRYC plays with teams of one calendar year. I'm sure whether that matters to you.
Starting with U5 - I think it's parents coaches with both leagues so whether you learn more would depend on the coach. For both leagues, I think for U4 they have professional coaches (golden boot, at least as of a few years ago).