Center of Gravity
In an RC, the center of gravity chiefly affects how the RC handles at high speeds, especially on jumps and turns. The lower and more stable the center of gravity, the less likely it is that the RC will flip or go off course.
With toy-grade RCs, center of gravity is of little concern because they really don't go fast enough to worry about it. With both electric and nitro hobby-grade RCs center of gravity is very important. Sometimes getting the center of gravity correct makes the difference between winning or losing in an RC race.
In my opinion, it may be marginally more difficult to have a steady center of gravity on a nitro RC compared to an electric because the electric RC doesn't have to worry about the constant movement of fuel in the tank. All the compontents in an electric RC are stationary and don't shift at all giving it a stable center of gravity and just possibly a slight handling advantage over the nitro or small gas engine RCs.
Weight
Just looking under the hood it's obvious that the nitro RC is going to weigh more that the electric. It simply has more parts sitting on that metal chassis. Although the high-grade aluminum and titanium are lightweight metals, they are still metal rather than the weight-reducing carbon-fiber plastics of an electric RC.
