At this point in time a natural gas vehicle would be more successful than an electric car. This is because electric cars are very expensive. Also, many areas do not have the needed outlets to plug an electric car into. Until there is more access to plugs and the prices are lowered they will not be very successful.
Natural gas vehicles are likely to be just as successful as electric cars but I do not think it will be more so. I think it will come down to what ever ends up being cheaper, and personal preference. It is a good chance they will be about even in the popularity.