I am having the same dilemma ..
My 6.25 Centrecab Supervee (6.5LOA) new build will be ready to fit the motors in about a month.....I will be mainly using it for spotlocking offshore but sometimes also cruising over reef flats, maybe some slow trolling with the downrigger and occasional barra lure tossing..
The main advantages that I can see by going the 105lb is that the motor will not have to work as hard in adverse conditions and the max amp draw of 105 v 80 is 41 v 56....so the 105 will draw around 35% less amps than the smaller motor working flat out....in theory does that mean that the 3 battery bank will get at least 35% longer runtime...?
On the down side (apart from the weight and space issue) of an extra battery and an extra $250 for the bigger unit and after enquiring about Solarking LiFePo4 batteries yesterday I was told they will operate in a 12 or 24v system but not 36v....
I was ready to go with the 105lb system yesterday except for discovering that......I really dont want the weight of 3x 45kg AGMs ..
So I need to do some more research on lithium's and 36v systems....