If you are going to rear face her I would go with the Radian....coming from someone who just ordered one today.
You said you like having the top tether...I am assuming you mean for rear facing, right? Because ALL car seats have a top tether for forward facing, well any you would buy, I am sure there are expired ones still in use that don't have it, because any seat without would be expired by now.
I don't like the complete air either. I checked one out last week and just wasn't into it. I was debating between the Radian or MyRide65 and one of the many reasons the Radian one out is that it can be tethered rear facing. I love that about our Marathon.
If you are going to keep her forward facing, it would make more sense to get a forward only seat, like the Graco Nautilus or Britax Frontier.
For me, I did turn DD1 just before she turned 3. If there were seats up to 40 and 45lbs at that point, I would have kept her rearfacing. Those didn't come out until she had already been ffing for some time and was well over 3. I will keep DD2 rear facing at least until age 4 because it is so much safer and seats have higher weight limits now.
If your only reason for ffing is A/C, there are ways to deal with that. Trying to remember what I read here, since it is not the season for that. I think the biggest thing is to point the vents at the ceiling and they will get to the rear facer better. Other ideas just to keep the seat cooler are to put ice packs in it when the child is out of it and to cover it with a towel or light blanket. We live in a very hot climate so we dealt with this DD1's first 2 summers. Oh we got the windows tinted and that helped a bit. Luckily that car was not doing so well and we got a van with rear a/c. Best thing ever...but there are much cheaper ways to keep you little girl cooler.