Oh yes, we loved our Radians! We recently graduated to boosters for at home use, and I still miss having those comfy car seats for car and airplane napping. I'll never forget that first time the kids tried to sleep on a flight without their car seats. It was a disaster! They've sort of figured how to manage at this point, but it was so nice to have that slight recline and 5 point harness when they were smaller.
Before we got our Radians, I hauled Britax Marathons on our travels. Talk about big and heavy. We took the Marathons to China, even though we knew we wouldn't be riding in cars with seat belts, just because I needed my 2 and 4 year olds to be able to sleep in their seat on the plane. 12 hours is so long! The GoGo Babyz wheelie thing was worth it's weight in gold with those heavy seats. I'd put the wheels on one, and stack the second seat on top. They aren't easy to install on planes, though, and I was so happy when we got the Radians!
We've never used the Kid Kares harness, but I've heard great stuff about it from other people.
I look forward to hearing what you decide, and hope you have a most excellent trip! Denver is such a kid friendly area!