Re: Radian R100, Radian R120, or Radian RXT?
Is she rear facing or forward facing? Does she sleep a lot in the car?
The R120 and RXT rear face to 45 lbs while the R100 rear faces to only 40 lbs. The R100 also has a lower forward facing weight limit, but I don't think that will be a problem with your little one. If you don't need the higher weight limit, then forget about the R120 because it's basically the same as the R100 except for the higher weight limits and the infant padding, which you don't need.
One important thing to note is that the head wings on the RXT will prevent you from getting full use out of the seats. Diono allows the child's shoulders to go above the highest harness slot but if you have the RXT, that can't happen because the head wings get in the way.
We have an XTSL (Sunshine Kids Radian with head wings), and I ended up buying a R100 for my 5 year old because the head wings were right at her shoulders. She has a lot more growing room in the R100. The only thing I don't like about it is that when she falls asleep, her head tilts farther to the side because she doesn't have the headwings to lean on. But we are just using a rolled up blanket to remedy that.
So, personally, I would opt for the R100 (though I do like the prints on the RXT much better).