If you go on lake Michigan then you will want to go an inch lower. the 19 may put you a bit LOW thein rpm range. You are only dropping from a 21 anyway. A lower pitch will help you troll a little slower and also help get you up and over the waves, especially when it gets rough fast.
If you are talking fuel mileage from A to B that is one thing but you will be idleing the majority of the time and cruising less. The prop is now part of your fishing gear and performs a duty that most others dont.
By the way, try painting the prop white or two blades white and one blade orange. Some believe it creates a flasher effect. stainless wont be as important where you wont likely strike bottom but it will also create some flash as it turns. I dont know if you want to spend a couple hundred more for a flashy prop.
I bought a 26 ft starcraft new in 1987 and knowing what kind of performance I wanted I ordered the motor accordingly. Believe it or not I got a 175 hp v6 I/O and never regretted it. I wanted it to troll about 1.7 or a little lower. It did that fine. I pushed 33 mph. You cant go fast when it is rough and you can enjoy the ride when it is calm and it trolled absolutely fantastic. I actually still have the boat but it is in storage. The props are 15 cupped and 17 uncupped. I put a picture of it below. today it sits in storage waiting to be rescued. I did a lot of work to it and needs a little elbow grease. the hard stuff was done already.
I got it that way because the v8 not only went with a 19 pitch but the gear ratio changed and it spun the 19 even faster. That would have made it harder to troll slow but would be good on Lake erie where they get out there fast and then shut it off and drift.
start with the 18 and see what happens. You might be surprised. By the way, since I am currently using a bass boat, maybe we can hook up in spring and I can teach you some fishing tricks to help put more in the boat or rig better for less money. I am still trying to get good weather to take the ranger out of Waukegan if I can,