Ahhh.. It depends. Are you towing the dink, or not. Lets say you're not. A small gas engine with an integral tank is easier to climb into the dinghy with from the big boat. Its lighter than the battery for the electric motor for sure; and you can do it one trip. Of course the Torqueedo motors are one electric model that has an integral battery.
Lets look at some weights:
A deep cycle battery for a trolling motor weighs about 70 pounds.
An electric trolling motor weighs about 20 pounds, so that's 90 total.
Go hi-tech electric with say a Torqueedo Travel series ( equivalent to 2-HP engine) and with the integral battery, it weighs 28 pounds
A four stroke Honda gas outboard with integral tank weighs 27 pounds with no fuel in the tank, so figure 30 lbs ready to go.
The first option is heavy and clumsy getting into the boat. The second and third options are easy to get in the boat.
Now, in use electric motors are nice in that there's full torque even at slow speed, which is nice for control around a dock or other tight spot. Gas engines though, can be run at full throttle, until the tank s dry; as a battery starts to wind down, so does an electric motors output.
Gas engines can be filled up from the a tank carried onboard the big boat, r perhaps veen siphoned from the main boat's tank in an emergency. You need a generator or shorepower handy to "refuel" your electric motor.
So you see it comes down to how and where you are going to use it, at least for practical considerations.
Aesthetically, there' hardly noise and no smoke at all with electric; small gas engines arent that loud, but noiser than electric and they do create some smoke, but not much.
Environmentally. Gas engines burn fossil fuels. Electric engines use a battery, which is charged by burning fossil fuels--and the manufacture and disposal of batteries isn't exactly "green." So in the big picture, they are about equal on this front.
oops--never answered the "size" component of the question.
Please tell me how big and heavy the dinghy is, whether you expect it to plane or not, and how many people you expect this wee craft to carry. Also, will you be using it where wind/ current is a factor? Or will it ply mainly calm waters? Is it your main boat? Or is it a pure tender: a means to go a few hundred yards from boat to shore?
Kevin has it right, your best bet is to go with the new style 4 strokes, super easy on fuel no smoke if you get the 4 stroke, no mixing oil and gas, and they are really quiet,don't have to recharge them,you are already burning fossel fuel to recharge, so no saving the planet there,lighter,compact,(no battery or charger to haul around), now as far as size goes its up to how much the tender will hold plus people and gear (total weight) now if you go electric you not only have the motor the battery but the two things with the battery is it is heavy and it also contains acid which can spill in such a small boat bouncing around can burn you and also it vents hydrogen gas which is extremely flammable( so no smoking) , good luck on your decision.