I am still a Mercury guy, but I am gradually learning that Yamaha is a better engine. I live on the big water. I know a lot of people who use Yamahas hard, and love the durability.
Depending on what you have and when you got it, there may not be much if any difference. Mercury has a long history with the Japanese makers. They first brought Japanese outboards to the U.S. with their "Mariner" line back in the day, probably made by one of the usual suspects....Yamaha or Tohatsu. In order to meet demand for small to mid sized outboards in the U.S., Mercury has bought Yamaha and Tohatsu complete units or power heads and sold them under their brand and may and or probably still does. There is virtually zero non commercial demand for outboards in Japan so its a win-win for them.
Back in the day when Bombardier bought out OMC, for a few years they sold Suzuki 4 stroke outboards under the Johnson brand before they phased it out.
All of this is subject to change based on who makes a deal with who. If I were in need of a new outboard I'd shop totally on price, maybe dealer. They are all good. Around here about all the commercial crabbers use Suzuki, likely because they are cheaper than the others.