Hellobee Boards

I have an RXT and I've never heard good things about it when it's used as a booster. I've loved it as a convertible. I had my first son in it from just shy of 2 until about 4 (flipped it when he was 3.5). Now I have our second son (who is now 2) in it RF. My first son is now FF in a Britax Frontier. FWIW, the Click Tight makes that seat a TON easier to install. The RXT is kind of tricky to install (worse RF, but still not the easiest FF).

My plan is to keep my older son harnessed until 6 or so in the Frontier, at which point we'll get a more compact booster and give that seat to my younger son FF, and then retire the RXT.

Edited to actually answer your question: I would get the Britax Click Tight. The RXT's real strength is as a convertible.

Yea I have heard that the belt positioned on the RXT can pinch and not be a good fit for everyone as a belt booster.

We actually have one of each (RXT and the Frontier CT) and honestly I think I would prefer another frontier anyways.

Here is one article about the RXT as a bad belt booster: http://carseatblog.com/14805/diono-radian-rxt/

I have loved the Radian RXT as a convertible seat for DD, but it won't last her into booster mode. She is tall for her age, and will most likely hit the height limit before she hits the minimum weight limit needed to use the RXT in booster mode. The Radian can probably get most kids through the age of 6, but they'd probably still need to be using it in the 5-pt harness mode unless they hit the 50 lb weight limit. Based on what you described, I'd go with the Britax!

Here's a review on the Radian as a booster -
http://csftl.org/diono-radian-convertible-car-seat-review/

Booster Mode
The Radian also has a booster function, with some limitations. The RXT booster limits are 50-120 lbs, however, the child must also have their ears contained within the top of the shell. Our 7 year old tester is only about an inch before his shoulders bump up against the headwings, at which point he will no longer be comfortable in the seat. We know that big kids need boosters until age 10-12, so the Radian definitely will not be the last seat that he needs.

The Radian did provide an excellent seat belt fit: positioning the lap belt low on the hips, touching the tops of the thighs, and the shoulder belt even across the torso. Received an IIHS Best Bet rating for 2013.