The Yamaha is a 4 cylinder.
The Honda is a 3 cylinder.
ETEC, forget ETEC. (for a lot of reasons)
Don't discount Suzuki either.
I prefer Yamaha because I have experience with them. I've run tons of motors, 90% of tiller steered and under 150hp, except that one F225 Yamaha that went to the lake the other day. Brrr..
Anyway, the weights aren't much difference between Honda and Yamaha. Both are good motors. What's cheaper? I don't have MSRP's in front of me. Yamaha goes out of their way to make it user friendly from a servicing standpoint. Worth mentioning. The 4 cylinder is going to be smoother and quieter running than the 3 cylinder-and likely be slightly more responsive. I have run one and while not quite as smooth running as the F70, it is a very close second. On the F70, it's nearly impossible to know it's running unless you can hear the pee hole water hitting the lake below (at idle speed). The F50 isn't much different-well because it's the same engine (but different cylinder head). It's so quiet, in fact, that when you remove the cowling and start the engine, you can hear the injectors "ticking" at idle speed. I used to have a video of it and used to play it in slow-mo for giggles. Honda has a 2.08 lower unit. Yamaha has 1.85. Advantage: Honda with the lower unit. Higher gear ratio equals faster to develop RPM to get out of the hole, but at the same time it's probably required since the Honda has a smaller engine which likely develops a little less torque (typical of Honda to be honest) at lower engine speeds than the Yamaha does. Yamaha comes standard with PT&T. Honda is available without-which is why they list their weight as lighter. It's all marketing tactics. Does Honda have a dealer closeby? You already found the Yamaha...do you like that dealer? Are they fair with you? Those are considerations though honestly it's unlikely that you'll ever have to use any of the warranty on either a Honda or Yamaha.
Haven't looked into Tohatsu much, though I probably should. They've made a lot of changes in the last few years. Good looking motors.
Also haven't looked too much into merc, though I have a 40hp 4 stroke in the shop now that's JUST outside the warranty period, and the exhaust cover gasket is leaking water, peeing it into the lower pan, fills the pan up, then the motor is "drinking" it's own exhaust water. Motor's got like 9 hours use on it. 3 cylinder like the Yamaha, I assume the 50 is similar if not the same block. It does shake a little at idle but not a lot. Noticeable though. The Yamaha F50? Pretty close to silky smooth.