I have operated both the Merc/Tohatsu/Nissan and the Yamaha...15 AND 20 HP models.
The Yamaha 15 and 20 are the exact same outboard. The carburetor is different. So you can buy a 15 for a few hundred less, and down the road when you get tired of only 15 HP, you can drop in a 20 HP carb and you'll have a 20. As far as I know, that is the ONLY difference. Even the CDI and electrical systems are the same part #'s.
I prefer the Yammie over the Tohatsu. The Tohatsu is cheaper, however. And they're available WITH power tilt (not power trim) which is a nice addition. They are both excellent engines. I do not like the fact that the Tohatsu's shift rod that runs from the powerhead to the foot is partially exposed. Easy to bend speaking from experience. Also the Tohatsu is a little noisier than the Yamaha is, the foot is a little noisy on the one I've used. The Tohatsu also is slightly cold natured and has to warm up a little before the choke is pushed all the way in. The Yammie has always started without choke even when stone cold. The Tohatsu also vibrates a little more than the Yamaha does, but it's not a night & day difference between the two as far as that goes. Having dealt with both engines, the Yamaha spends a lot less time in the shop and the folks at Yamaha always are willing to help IF there is a problem (rare), oh and I must admit, they are also very professional. You pay a little more up front but IF there is a problem down the road, even if they're out of warranty, customer service is willing to assist in one way or another.
I believe the 15 and 20 Tohatsu are the same motors as well, though I have not researched them much. I know that Yamaha 15 & 20 are identical...even the cowling is the same (different decals). Same weight, muffler, exhaust, intake, head, cylinders, pistons, powerhead part #'s are the same, foot is the same, just different carb.
As far as 2 stroke vs 4 stroke, the ONLY difference is that the 4 stroke has slightly less torque in the midrange. I mean SLIGHTLY. Maybe 1 lb-ft if you're lucky. They're both still the same HP. The only other difference, obviously, is the weight and that can be compensated for by moving the load around in the boat. Maybe going to a lighter battery or moving the fuel tank. The guys who hate 4 strokes are those die hards who use their boats for bulldozers in the flooded timber, and every pound is crucial. Also tilting the 4 stroke isn't as easy as the 2 stroke (weight) but if you unlock the tilt lock and put it in reverse, it will come up easily. I prefer the 4 stroke mainly for cleaner running and much better fuel economy. One of the holes I run to on a local lake is about an hour's run wide open. With my 25 4 stroke Yamaha, I can make it there & back on about 2 gallons. With my friend's 2 stroke 25 twin carb (25ESH), it's 6 gal. When we get back, the tank is almost empty. Identical boats Duracraft 1542's. His runs 28 mph max, mine's 27 but he's got a 9 7/8x12 prop and running 5800, where mine is a 9 7/8" by 10.5 running 6200 (I know...too much...working on that). Not to mention I can idle ALL DAY confidently with no smoke, no roughness, no plug fouling, no loading, and no roughness whatsoever. There is a river close to use where the launch is about 5 miles from where I fish, and the river is skinny and rocky with current and I have to idle those 5 miles as running at any speed would be destructive. I have a boat not a rock crusher.