I don't have any experience with Tohatsu engines, but I found a test in the Swedish powerboat magazine Vi Båtägare
. They compared Mercury 4-stroke, Tohatsu Orbital and conventional 2-stroke Tohatsu. Test boat was a 4.7 m long GRP boat, weighing 280 kg without engine.
The Mercury is the fastest engine, some 2.5 knots faster than the Tohatsus on average (3.5 knots faster at WOT).
The Tohatsu Orbital is the most fuel-efficient engine. It is interesting that the conventional 2-stroke Tohatsu is more fuel-efficient than the 4-stroke Mercury at speeds between 20 and 24 knots!
The acceleration is very even, but the 2-stroke Tohatsu has a slight edge here.
The 4-stroke Mercury is, as expected, quieter than the other two engines, especially at lower revs.
The Mercury is heaviest at 107 kg, the Orbital in second place at 83 kg and the 2-stroke is lightest at 67.5 kg.
When it comes to price the 2-stroke is obviously cheapest. It costs SEK 42000.00. The Mercury costs SEK 57500.00 and the Orbital costs SEK 66990.00.
So, it would seem that the 4-stroke is the way to go. This is also the conclusion in the article.
Hope this helps!
Divide the amount in SEK by 11 to get US$, and divide it by 15 to get UK £.