Hi kate-
No problem-- here's my input.
I have the Radian R120 Shadow:
Yes, it converts to a booster and is weight compatible up to 120 lbs. My friend has her 8 year old and 9 year old still in their Diono with the 5-point harness. I'll probably do the same since the kids can buckle themselves in then.
My friend has the radian RXT which is the same but has wings around the head. She says that her kids like to sleep against the wing flaps. Alex prefers to be more open and I read that the flaps may interfere with shoulders on tall kids so I went with the non-wings (r120).
Between the storm (fleece) and shadow (nylon), we prefer the shadow. My friend had 1 of each and the fleece definitely started looking old, worn, and tattered over time. If you do research, one of the materials had less fire retardant in it but I can't remember which. I still prefer the shadow.
The Diono needs a foam cushion accessory to go rear facing. And it also takes up a lot of space rear facing such that the front seat can't recline as much. Once you go forward facing this isn't a problem.
I, personally don't think the r120 is good for kids under 1 year or under 1 1/2 years because it has no side protection. So I have Anna (11 months) in a britax rear facing and will transition her to the Diono when her neck seems a bit stronger, maybe around age 2.
The Diono has a steel frame which makes it more sturdy but also makes it heavy. It's not a car seat to travel with on the airplane.
And the Diono is known for having a narrow profile. So you could fit two Diono car seats in the back row and still have space for a passenger in the middle.
And I often see the Diono on sale. If you wait a little bit, somebody always has a sale.
Hope that helps-