I'm not a tech but I'll chime in briefly on my latest travel experience as my child is roughly the same size as yours, she just turned 4 and is about 35# and 42" tall.
In the past, I've traveled with either a Britax Roundabout or a Marathon.
After much consideration, on our last trip I took both a Radian and an RSTV. My child is at the minimum weight of the RSTV and I played with it quite a bit before we left so that I was comfortable with the fit and installation. It was time well spent, I was not happy with my original efforts with the RSTV so I'm glad I figured it out at home, not after a long travel day.
We used the Radian on the airplane and I was really delighted how well it went. I'm a single mom and purchased the carry straps to carry the Radian as a backpack. It is surprisingly comfortable for such a heavy seat. It really is easier to get on and off the plane and around in general because it folds. Again, I spent quite a bit of time playing with the folding mechanism and practising adding and removing the safe stop thing because I knew I'd have to do this in a tight space on the plane in short amount of time. It really installs on the plane like a dream. When I forget to use the recline, my childs head slumped when she slept, so I do recommend using the recline for added comfort for your child. I don't think I'll be flying without a car seat any time soon. My child is so comfortable on the plane in her own carseat, I don't think she'd sleep without it. On our way to Alaska, I actually read a entire novel while she slept on our 5hr45min flight.
Anyway, on land I used both the Radian and the RSTV. I bought a backpack large enough to accomodate the RSTV and carried it with me all day every day just in case we decided to grab a cab or whatnot, and took the Radian when I knew I could use that. We were on a cruise and rode in a couple 18 passanger type vans but mostly small 24-40 seat busses, most had seat belt only. As I said, I'm not a 'pro' with the Radian, but I had no problem installing the Radian anywhere. The times I had to install it where there were strangers in the vehicle, everyone was helpful and understanding and I never felt rushed.
We already had the Radian, but did purchase the RSTV for this trip knowing we'll get lots of use from it in the future. I use it for my niece and will use it for preschool field trips and future travel needs.
All that said, I probably wouldn't hesitate just lugging a Frontier along on a trip. I'm a believer in using what you have. I've been traveling with carseats for 15 years and it is never as big of a hassle as I think it will be. If I can do it as a single parent, I think anyone can.
HTH