The Tohatsu, Nissan & Mercury's are all Tohautsu's. Part for part they are virtually identical.
That being said the Mercu-hautsu-sans weigh about 41-42 lbs. and the little Suzuki 2.5 weighs 29. The difference between a 2.5 and 3.5 will be hard to notice. If it were me I'd be buying the Suzuki, for the weight and the fact that it's still water cooled, but they are all very reliable motors and you can't go wrong with any of them! Don't pay the premium for the Merc as they all carry the same warranty.
I knew that the Tohatsu, Nissan & Mercury's are all Tohautsu's. I wasn't sure of the noticeable difference (or lack of difference) in power between 2, 2.5, 3.5.