is basically the Merc Optimax system by another name. (IIRC). So, digital fuel injection, lower fuel consumption (upto 40%, less emissions, more too go wrong!, etc etc.) I would think it would be lighter than the equivalent 4 stroke and probably give similar fuel consumption figures. That being said a 50hp conventional 2 stroke is not going to be that thirsty - The joy of things like Optimax are when you get too the larger hp ranges (150hp+) where fuel consumption is a real issue!
I run a 150 Opti and its great. No experience of the Tohatsu though.
Check out the article from Andy Clarke (Rib internationals outboard guru) http://www.bosunsmate.co.uk/tohatsu.htm