The best booster is going to be the one that fits your son the best, you shoul try to find a store that will let you try them out in the car. You want the lap belt to be low across his lap and not ride up on his tummy (as that is a higher risk for internal injuries. At his age definitely get the high back. Is there a reason you want to move him to a booster? Has he outgrown his harnessed seat. A child in a booster is responsible for their own safety, does your son have the maturity to stay in position, and to not put the shoulder belt behind him our under his arm? If the answer is no you may want to consider keeping him in a five point harness longer (closer to age 5 or 6.)