I really think it depends on preference. There's good and bad points on both sides of the class 9 spectrum, if you want to call it that, lol. For instance if you want a single seater that's just less money that you would have to spend on another racing seat, safety harnesses, etc. More tubing is usually required to make a two seater and more tubing = more $$$.
So in general, a two seater would most likely be more expensive than a single seater, but there are some other advantages of having a two seater. You can have a spotter, or rather, an extra pair of eyes to keep a lookout for when you're racing, another person to enjoy the experience. (as mentioned in a previous post) It would also be good for showing people "the ropes" in off road racing, if they've never had experience in a car. Plus you could charge for rides, haha, just kidding.
So yeah, for the most part it really is based on preference. There may be some weight issues between the two types of vehicles, but I've heard that there's certain "adjustments" for balancing out the field.
