Two things
First you can get 36v in higher thrust than 24v
Second....you get more hours run time with 36v
I use a 24v 70 lb thrust on my boat and I can pull crankbaits at 1.8 mph continuous for 8 hrs and still have some battery left.
The guys with heavier boats than mine use 36v 110 lb thrust to get 8 hrs time out of theirs pulling crank baits.