Both brands are great, both producing 90HP at similar weight. It would really come down to who can do you the best deal $$ wise. You would be pleased with either.
On my previous boat, I had a merc 4 stroke 115HP. Nothing but praise for it. Run for 3 years, without an issue and looked like new when I sold the boat. Im certain that Yammie would be exactly the same.
Start negotiating with dealers for both and choose the best price - brand, reputation, longevity, performance, quality, fuel efficiency etc are essentially equal - it will come down to price