...there is no reason at all to opt for the 150 if you are not going to use the extra power it has over the 115...
Unless you want to be able to cruise at a lower power setting, increasing reliability and longevity. Amortized over the life of the engine, the initial higher cost now is probably going to be less than having to repower sooner because the powerhead has been working harder. You'll also get more on resale, though you never want to bank on that.
The common rule-of-thumb is one gallon per hour per ten horsepower.
I run twin 150s (2 yrs old, 900hrs each) - if the OP can cruise at 60% throttle, he will be burning about 6 gal/hr.
The 115 running at 80 or 90% will burn more...but probably not significantly.
All that being said, with the hours most people put on in a year, it won't really matter.
You didn't hear it from me, but keep an eye on Merc at the Miami boat show: the 150+hp non-Verado hole in their lineup is finally being plugged.
When people ask me about repowering their boats, my stock answer is to buy the most power they can afford up to what the data plate says.
The fact that the mfr specs a 115CT for the base engine suggests they're trying to hit a price point with a smaller powerhead.
Generally speaking, never take the base power option on any vehicle, especially boats.
Reserve power is peace of mind.