Just thought I would update with a comparison of seats for anyone looking for more info. Although I could not find the specifics of whether the seats can be attached with lap belt only. Our Cardinal back seats have shoulder straps as well as the lap belt.
Frontier XT:
Seat weight 19.6lbs.
Forward facing weight and height up to 65lbs and 57".
Coverts to no-back booster (so using just the plane harness, not the seats 5-point) 120lbs and 65"
More of an upright seat (better for looking out windows, not as good for sleeping) and has built in cupholders.
Evenflo Sureride DLX/ Titan 65:
Seat weight 14lbs
Forward Facing weight and height up to 65lbs 54".
Does not convert to booster.
More of a relaxed reclined seat (good for sleeping, not for looking out windows), no cupholders.
Diono/Sunshine kids Radian XT:
Seat weight 32 lbs
Forward Facing weight and height up to 80lbs and 57"
Converts to booster (back on, just removing 5-point harness and uses planes harness)
Upright seat with attachable cupholder.
We haven't bought car seats for our plane yet. We have the Diono's in our car, but are thinking we will go with a lighter seat for in the plane. My guess is we will go with the Frontier as it is has a few inches more height, and will grow with them after they are 65lbs.
The only problem I see with this list is that the Frontier does NOT convert to a backless (no-back) booster. It's a very solid harnessed seat that converts to a high-back booster.