I'll take a shot
I had an early 1980's 9.9 on a 14 foot boat {long ago} and I figured a little more than a gallon an hour. If there is room for the larger tank and I was useing it as you descibe than the larger tank is the one I would pick. If I had the smaller tank I would be lifting it up to measure how much fuel is left and would be worried about running out a lot. Lets face it, One gallon of fuel left in a three gallon tank would be a 1/3 full but would seem pretty empty. Good luck, Jack P.S. I had that 14 foot boat on Casco bay and Sebago, I sure miss Maine.