The 15 or so extra pounds a 8 or 9.9hp fourstroke has over an older two stroke, or a smaller 4 stroke really isn't much as far as weight in the boat. Its more of a issue if you have to tote the motor around from the garge to the boat a lot. As far as balance in the boat get or make a longer fuel line and move the fuel tank up front. If you have an electric on the boat you can also move the battery around to help level the boat.
I have a 6hp tohatsu on a 12 ft V-hull and while I agree its got a lot more vibration than a twin cylinder, particularly at low ( trolling speeds), as long as you primarily using it to go from point A to Point B a 6hp would serve fine. If your going to troll a lot then I would definitely go for the 8 or 9.9. Even though the 6 does vibrate a bit, it doesn't load up like a 2 cycle and you can troll with it, its just not smooth.
