I think you'll find that an electric motor is much more of a pain in the butt than you think. Not only is it vastly less portable, you have to fight with cords (wrestling with a 100 ft 6 ga extension cord isn't much fun), you can't use it in the rain.
The reason an electric motor seems like it gives better performance with lower horsepower, is because they produce a constant level of torque at any RPM. A gas engine produces low torque at low RPM's, then climbs and peaks, and then begins to trail off at high RPM's. Horsepower is simply as measurement of the rate at which torque can be delivered. 5 hp out of both is the same, although the toque output may be different, because the RPM's may vary. You need more information to truly conclude that the electric motor is better. In this case, the only way I can see it working out to your advantage, is if you left the extension cord laid out, and left the splitter in one place in your yard and never intended to use it anywhere else. Otherwise, the greater portability, lower complexity, and not having to deal with cords, outweigh the slightly higher torque at low RPM's of an electric motor.