Okay, so the difference between the R100 and R120 is weight limits. The R100 has a 65 lb harnessing limit and a 100 lb booster limit; the R120 has an 80 lb harness limit and a 120 lb booster limit. The RXT is the R120 with headwings that are supposed to offer some side impact protection (there is no standard for testing SIP in North America, so whether or not it actually makes a difference isn't measurable).
I have older radians, from before the switch to Diono. I have a radian 65 (same as the R100) and an XTSL (same as RXT) and honestly, my kids greatly prefer the 65 because the headwings get in their way. And they ended up having to stop using the seat a touch earlier than allowed by SK/Diono because the headwings interfered with shoulder space.
As far as weight limits are concerned, they are all but irrelevant. The vast majority of kids outgrow carseats for height light years before hitting the weight limit, so I personally would not pay extra for the added weight limits. My oldest hit the height limit to harness in the radian at 40 lbs, although she is tall, skinny and long-torsoed.
Also, a word of warning that the booster mode on the radian is all but useless, so you will need to plan on buying a dedicated booster once the radian is outgrown (this would not deter me from buying radians, though, because they're fantastic seats). The seat is outgrown height-wise at the same exact time for booster and harness modes plus there is a 50 lb weight minimum to use it in booster mode, so my daughter outgrew it for height a year and a half ago at 40 lbs and is still nowhere near the 50 lbs required to use it as a booster.
In case you weren't aware of the rules for when a seat is outgrown for height:
RF: A seat is outgrown for height rear facing when there is less than 1" of hard shell (not including headrest for seats that have moveable headrests) above their head. It is also outgrown if the manufacturer has stated a height limit for RF and the child has hit that.
FF: A seat is outgrown for height forward facing either when their shoulders go above the top harness slots or when the tops of their ears are even with the top of the shell, whichever happens first.