I'm jumping in with no helpful information, but a question about the Pria, since you keep saying it has more room. And I guess a question about the 25" reference to the Radian.
I'm *assuming* that 25" in the Radian means there is 25" from the bottom of the seat to the highest harness slot? Or IS that 25" to the shell?
Because I have a Pria (and have never had trouble installing it?), and when a lady asked how tall the shell was when fully extended, I measured either 24 or 25", but that was to the top of the shell, not to the top of the harness (and I specified that to her, since that was what she asked). I don't know exactly how high up the top harness slots are.
So I was just wondering how long you think the Pria would last rear-facing? And I'm probably asking it poorly... Sorry!
*edit* I mean how tall the headrest is extended, not shell. Haven't had enough coffee this morning!