Well, you could always buy a R120 and buy an RXT cover for it. It will fit just fine (but not the other way around; you can't put a R120 cover on an RXT).
Both seats will last the same amount of time rear-facing. It's when forward facing that the head wings start becoming a practical space-limiting feature. Whether it matters to you really depends on how you're going to use the seat.
A child should be able to use an RXT until the shoulders are even with the top slots of the seat. That's a really long time, and should get most kids - even tall ones - to kindergarten. Most car seats are outgrown when the shoulders go over the top harness position. Diono says it's OK to keep using the car seat until the tops of the ears go over the top of the Radian, though. And this is where the grey area comes in. The head wings adjust to just above the top slots, and can't go any higher. So when the child's shoulders go above the top slots, they start bumping into the head wings.
But many parents aren't comfortable with continuing to use the seat after the shoulders are above the top slots anyway. For those parents, the space limitation of the head wings shouldn't make any difference.
I did replace a "winged" model of Radian with a "wingless" model some time ago when I was babysitting a family member in kindergarten. About the time she hit 6 years old, she got too tall for the wings in the Radian and was getting uncomfortable (by the way, she was probably a head taller than all the other kids in her kindergarten class as far as I could tell). I would have been fine moving her into a booster at that point, but for space reasons I couldn't squeeze a booster in the seating position I needed her to ride in. The wingless model fit her just fine for the duration of time while I was babysitting her... I haven't tried her in it recently (she's almost 7 now) but my guess is that she might still barely fit in the Radian without wings now. I'd probably guestimate you can get an extra year of use out of a model without wings... if you really need it. But most families will be ready to move their kids to boosters by then anyway.