The Britax Marathon/Decathlon/Boulevard seats are often recommended because they not only have a 65lb f/fing weight limit, but they have taller backs and top harness slots and will accomodate the average child until about age 5, when it is much safer to move from a harness seat to a bpb. The gov't recommends waiting until minimum 4yrs & 40lbs to move up to a bpb but CPST's generally agree 5 & 40lb is much better because kids are (generally) mature enough at that point to stay seated properly 100% of the time in a booster w/the lap/shoulder belt (BPB).
The Britax MA/DC/BV seats have 17.5" top harness slots compared to most of the 40lb limit seats which max out height wise at 14.5-16" torso height (when the shoulders go past the top slots the seat is outgrown by height fwd facing). That extra couple of inches can give you a couple more years before the seat is outgrown.
The Britax seats also have built in lockoffs (great for seatbelt installs), and can be tethered in r/fing mode (up to 33lbs in Britax seats - r/fing is the safest way to travel and the AAP recommends r/fing until the child meets the height or weight limit before turning it f/fing). Only Britax and Sunshine Kids currently allow the seat to be top tethered in r/fing mode. (It's an anti-rebound feature).
I posted these links on your other thread, but these two seats are other options in this category:
Sunshine Kids Radian also
www.skjp.com Read this thread for all the pro's and con's:
http://www.car-seat.org/showthread.php?t=6263
Fisher Price Safe Voyage Deluxe
made under license by Britax for FP. Same size shell, and 17.5" top harness slots. R/f to 33#, ff to 55#,
no HUGS or r/fing tether.
In May, Recaro is introducing a convertible that will r/f to 35#, f/f to 70#, 19" top harness slots(!) If this seat lives up to the Recaro reputation, it might be an excellent choice if you can wait for it. Cost $219 & $259
Edit to add: For flying we used a Cosco Scenera 5-pt ($40 @
Walmart/Target/Kmart). Unfortunately ds outgrew it by height at 34mos but for that price we definitely got our money's worth out of it. Now we travel with a combination booster seat instead. The BV stays at home. It's airline approved, but it's just too darn heavy!

Another option is attaching a Gogo Kidz Cart.
Hth!