I researched this awhile back because my needs are similar to yours.
All I wanted was a unit that could keep me going straight while setting lines or when bringing in a fish.
At the time, The Lowrance seemed like the best bet due to its relatively low price.
What I discovered back then that nixed the deal was that the Lowrance I was considering would not hold me true at trolling speeds.
Apparently, the unit could not detect the subtle changes in direction at trolling speeds, so I had to abandon that plan.
Admittedly, I did this research several years ago, so it is quite possible the newer units might work now, but I decided to pass on the idea.
My guess is that what you are paying for in the more expensive units is a much better heading sensor and a faster (higher capacity) pump.