Ive learned to stay away from Yamahas in the boat rental business, toughness is not one of their strong points.. Last year at one of our places they had to replace 8 lower units, all of which were Yamaha.. Ive seen Mercs come back with literally all 3 blades half gone, half the skag missing, lower unit still kicking... If you even look at a rock, stump, etc. wrong in a Yamaha youre probably going to need to replace it!!
That being said most people love them and I never really hear bad things about them.. But from my experience what ive seen first hand, Merc hands down wins on a toughness standpoint..