For my son's safety I kept him in a 5-pt harness when he outgrew his convertible car seat. Keep in mind that he is now 5, and many of the convertibles can be used until the child is roughly 45-60 pounds. My son is 5 yrs old, and our seat "expired" for his weight and height at 40 pounds.
That being said, I went to carseat.org and started reading and asking questions. They provided tons of information, one of the most salient/meaningful points was that my child would be safest in a 5-pt harness for as long as possible. The benefit of going to that site, is that the moderators are all safety car seat technicians, who are trained and certified in inspecting and installing car seats of all kinds. They are not representatives for any brand or manufacturer, and most of the them are parents. They can provide you information about which seat would be most beneficial for your child (given that you provide them with his height, seated height, weight, and age). If you provide them with the year, make and model of your car, they can even give you the information regarding the seats safest for your child, along with those that will best fit in your car.
We went with the Britax Regent for my son (he was 3.5 when we purchased it), and Britax had not yet come out with the Frontier... and we couldn't wait! I probably would have purchased that, had it been available! I didn't want to make another purchase to cover the 65-80 pounds for my vehicle. For my husband's car we got a Graco Nautilus, which will keep our little guy in a 5-pt harness up to 65 pounds. We're still a long way from that (weighing in a 48 pounds currently). FYI, both provide excellent head support when he sleeps (even better than his convertible did when he was younger). He has never complained about not being able to see.
Something to keep in mind as well, is that it was brought to my attention by a certified installer, that many states are exploring legislation to keep children in 5-pt harnesses until they reach a specific age and weight range, and prohibit use of booster seats.
Go to carseat.org, get their opinions! They give awesome advice. Research and try to avoid having to purchase tons of seats for your child. Get one with room to grow that will keep your precious one safe, comfortable, and will fit in your vehicle(s) and lifestyle!
Here's a tip too... many stores will allow you to try it out in your car, and if they do have your little guy sit in it and see what he thinks. You can make an on-line purchase (e.g. Target, Radian-80) and if it doesn't work out, you can return it at the store (and avoid a return shipping fee).
Good luck!