Don't try to over think it. Use your motor the way it feels best to you. I don't think it matters a whole lot what speed that you use. Get the battery with the most reserve power that you can afford. That matters more than your motor speed.
If your TM has speeds 1 - 5 and is not variable speed, it should use a speed coil to reduce the voltage going to the motor and control your speed. From what I have read, this basically means that is does not matter what speed you run your motor, it should run the same number of hours on a single battery charge at speed 1 as it will on speed 5. With a speed coil, your TM will pull the same amperage at speed 1 as it does at speed 5. The difference is that at speed 5 all the voltage is being used to power the boat but at speed 1 the speed coil is acting as a large resistor to reduce the voltage to the motor by burning of part of the power as heat and the boat goes slower. I have to assume that running at high speed will use slightly more power because the resistance of the water at the higher boat speed should cause a little more power to be drawn from the battery but the difference is probably not important. At speed 5, you will cover more distance on a single battery charge than running at speed 1 because you get the same amount of time from a battery charge regardless of the speed setting and speed 5 is pushing the boat faster than speed 1 during that same time period.
A digital motor always sends the maximum voltage 12v or 24v to the motor on each pulse. The more off than on time in each pulse, the slower the motor runs because the power being turned on and off 20,000 times a second. With a digital motor very little power is being burned off as heat since each bust sends a full 12 or 24 volts to the motor on each pulse.
Based on the assumptions above, I usually run my TM at speed 5 to get from point to point. Then when I am using the TM for postioning the boat or maintaining a position on a brush pile, etc, I use speed 3 or some times 4 and turn the motor on an off with my big foot switch to maintain my poistion. I would use speed 5 all the time but the motor jumps too much when I turn the Big Foot switch on and off with the motor set at speed 5. The digital motor controls this with soft start and rapid power switching. I almost never use speeds 1 or 2. I feel I get more time on the water using my big foot switch to control my speed with a speed coil motor instead of turning the handle up and down. This is probably hard on the motor and will reduce it's life expectancy in the long run. By then, I may be able to justify a digitial motor.
Because the elecricity is turned off, more that it is turned on, a digitial motor can give you upto 5 times the time under power on the water as a speed coil motor on the same battery fully charged if you are running at the slowest speed. Check out PWM motor control on the internet.
I have to assume that if a digital motor were constantly run at full speed and a speed coil motor were run at full speed, they would both give you the same amount of time on the water for a single battery charge.
In the article below, MinnKota says that a speed coil is like holding your accerator your car all the way to the floor and using the brake to control the speed. A digital trolling motor is like using a throttle to control the flow of fuel to the car.
https://www.minnkotamotors.com/_ui/...o_banner_faq-item.aspx?id=147&terms=maximizer