I have been out of the water for a couple of years and my experience was not with 50s, so take this for what it's worth.
Yamaha makes the Mercury 4-stroke engines. Or they did - not sure if anything is happening yet with that Mercury plant in China. If I recall correctly, Mercury has a lower (published) operating cost because they have a longer service interval and cheaper spark plugs,even though it is the same engine.
I would go with the better dealer, or what your mechanic prefers, since that's what will matter if something goes wrong.