They all meet the same safety standards. Other than that, features are what make the difference. Since he is already 5 months old, I'd look at a convertible one. You'll be able to keep him rear facing longer and use the seat for several years instead of just a year.
What features are useful?
A cover that is easy to remove and wash. Babies have diaper blowouts. Toddlers have potty training accidents. Toddlers and preschoolers also spill drinks and food on their seats. A cover that can be removed without taking the seat out of the car and without removing the harness from the seat is GREAT.
Built in cupholders are great too. These can hold bottles, cups, and tiny treasures from trips to the park.
at 5:11 PM on Apr. 1, 2009