We have a high back booster for our four year old. The difference is in how they adjust the shoulder belt. We like the way it adjusts the should belt to his should level.
But I am not an expert. As a parent of three (the 4 year old being our middle child), I do recommend going with the one that is going to work best for both your child and your car while still allowing the most use. In Pennsylvania, they have to be in a carseat until they are 8 years old. We went with one that can be used up to 100 pounds just to be sure it would work for as long as we might need to use it. (Although, I'm sure that the one that only goes to 80 pounds would probably have been sufficient. I just wanted to be covered in case they changed the law AGAIN.)
Anyhow, I hope this helps you out a bit. If you have any further questions about how we like our seats, send me a message. I'd be happy to answer any questions you may have on our experiences with carseats. (Our oldest is almost 9 and our youngest just turned a year old.)