I have both and they have their advantages and disadvantages. The electric motor develops maximum torque at power application. If you are trying to get across a busy intersection, you can be at 15 mph in a matter of a couple of seconds with no pedaling. This does take a toll on the battery life. As others here have said the gas engine is like starting off in 3 gear on a car. Slow acceleration and you have to pedal to you get some speed then apply the power.
The top speed is a little better on my gas bike.....about 22 mph with a Stanton friction drive and 1" roller plus I can ride all day if I want to. I use the electric bike for trips under 6 miles round trip and it makes almost no noise. Very pleasant to the people around me. I have massive hills where I live. I can't even tell you what top speed is on level ground because around here what I consider level would be a hill to alot of you.
I don't think I could make it without both of them.