The Yam is 27kg plus oil plus fuel (max 2 kg) and no extra walk back for the tank.
Merc, 25kg, plus oil and walk back for the 10kg tank and fuel line.
Both motors are 4hp derivatives, no advantages there. Check this.
Yes, the current Yam (cream) livery is poxy. So what?
The Yam can be transported on any three sides - a major bonus, IMO.
I have VERY limited experience with fuel burn on small (mouse fart) engines but an afternoon of empirical testing with my 9.8hp 2T Tohatsu showed that I could bomb about at WOT for feckin' ages (about an hour) and burn 2.5L. Translate that into 6hp 4T terms and that's a decent range.
Look at camping fuel vessels, i.e. 1L aluminum bottles. One will fit in your pocket and double your range/safety margin.
Think about it.
However, it's an entirely academic argument. You need a 9.8hp 2T