The overhead shield pose an unacceptable risk of head injuries for smaller children and are easier to escape for older ones.
http://www.cpsafety.com/articles/fivepointshield.aspxIf it fits in your car, I'd go with a Marathon, which should fit until your child is six or so, at which point you can more to a inexpensive booster. However, the best people to ask are at the parentsplace.com carseat board or at car-seat.org both of which have helped me pick out my son's seat (a Britax Husky, which we love) so I would ask them for advice. Most of the regular posters there are CPS (child passenger safety) tecnicians or instructors, so they know thier stuff.
BTW, it's best to keep your child rear facing as long as he's able to do so within the guidelines of the seat.
http://www.cpsafety.com/articles/StayRearFacing.aspx explains why. (As an aside, most of the kids in the gallery there are the children of CPS techs at that site and almost all of them are in Marathons or Wizards which use the same shell. I think that says all you need to know about that seat.)
PS Feet hanging over the shell don't mean the infant seat is outgrown, now will they mean a child in a convertible seat should be moved forward facing. The determining factors are weight (and at 20 lbs he isn't too heavy yet for any, although he's at the limit for most of them) and height. If his head is within one inch of the top of the shell (not the fabric cover, which should extend beyond that a ways) then he's too tall for the infant seat. The seat may also have a height limit expressed in inches, for example the ever-popular Graco Sungride has a height limit of 26".
No matter what seat you choose, be sure to have it inspected by a CPS Tech, because 9/10 seats are used incorrectly and it's vital to make sure your child's isn't one of them. You can find one here
http://www.nhtsa.dot.gov/people/injury/childps/Contacts/index.cfm or here or here
http://www.safekids.org/ I'd stay away from Cosco seats because the straps twist up, the instructions are bad (the Alpha Omega has got to be the most freqently misused seat in the universe) and thier ristory of recalls is awful. My first choices would be Britax, Combi, Graco and Evenflo, in that order based on the quality of thier seats and thier ease of use.