Not sure if you're talking about a bow mount or a kicker but, since I've got both, here goes...
I have an 80lb Minn Kota Riptide on the nose of my 20. It's the one with the auto-pilot on it, 80APST or something like that. It's a 24 volt system. I can run it all day fishing in 2-3 mph currents and only lose ground at the end of the day. If I'm using it for boat control and casting shorelines the 80lb motor is enough to power the boat for a couple of days of all day fishing without recharging the batteries. The AP feature is awesome, especially for working shorelines. It's really nice to fight a fish without having to worry if your boat is going to drive into a dock or something. I wouldn't buy a motor that didn't have it. It DOES tend to wander around on me at the higher thrust levels, but what doesn't?
I also have a late 70's Johnson 15 on a kicker bracket on the port corner. I tie it to the main engine with an EZ-Steer and, for now, just regulate the throttle at the tiller. It isn't the best set up. I'd rather have an updated 4 stroke like a Yamaha T8 that would do better on the troll and allow me to control from the helm. It also throws off the balance while underway but tabs would fix that pretty easily.
Hope that helps...
Scot