I'm going to be the sensible one. How long have you been wheeling rc cars, and how good a driver are you? Simply put, if you haven't much experience go with the 1/10th scale. 1/8th electric buggies have allot of power and for someone starting out you will do more harm to your driving than good. To be honest for starting out I would recommend a 1/10 stadium truck, as they seem to be the most forgiving 1/10th class.
Now, if you have been doing this awhile, then it is a flip between 1/10th 4wd, and 1/8th. They are both extremely fun classes. 1/8th cost more, but is more durable, and can handle rutted tracks better. I think if you are a good driver, then just get which ever has the biggest turnout at the track you are planning to attend.