We went to 20's on beans a couple years ago.

Had done drilled 10"  a long time ago, then 15's, then was straight 30's for a while.  

Probably really went to 20's because it was the easiest way to get narrower rows in a 60' width.  Older Kinze 3700 planter.  Price was for sure right.

Can't really claim a proven yield advantage or disadvantage against 15's.

If you drive straight with your sprayer the wheel goes between the row and you can't see a track which is hard to do on 15's.  Row cleaners work ok on a single bar for no-till.  A bit tight between units if you want to work on them.

Canopy is a few days later than the 15's, you have to decide if that helps or hurts.  Seems to matter less the earlier you plant.  I guess a guy could diamond row or shift over 10" on RTK if it got really late.

You get 33% more push in the row vs 15's if you have worked soil that wants to crust over if it sprinkles, which is half our acres.

You don't have to carry around interplant units on your corn planter.

Seems like you can see rocks a bit better than in 15's come harvest time.

I can't much prove superior agronomic outcomes but for operations 20's are very handy.  I think that will be the way to go on corn eventually but I have to be a better corn farmer before crowding in 30" rows becomes my yield limiting factor.