I'm thinking you should try some drift socks first, I'm sure with the right set up you can get your speed down.
Personally, I like the electric but like Scott said you have to charge the batteries which can be a pain if your gone for weekend and can't charge the batteries. With my Minkota Terrova (101lbs. bowmount) I can troll anywhere from 0 to 3.0 mph, it also has an autopilot. The batteries will typically last me 8-12 hrs. depending on the conditions and speed your trying to maintain (with a 21 ft. Triton). I have heard only good things about the engine mount trolling motors but have yet to try one for myself.
As for the gas kicker, I only use mine @ certain times of the year for very specific applications. The best one being getting back to the launch when you have a failure 30 miles away