If you do both salt and fresh, why not just use the salt water motor in fresh water. I wouldn't go fresh motor in salt though.
According to the Minnkota site, Riptide saltwater TMs have "Advanced Corrosion Protection", so it should be more durable in the salty water. And they have a sacrificial anode.
The other difference - a few hundred clams. On iboats - cheapest Riptide 55# is $519, fresh water 55# is $240.
Is a salt water trolling motor better sealed? And it that is the answer, why wouldn’t everybody choose a salt water trolling motor then?
If you do both salt and fresh, why not just use the salt water motor in fresh water. I wouldn't go fresh motor in salt though.