CLIFTON and Challenger are the similar 2 shoes int he Hoka line. I believe they are the same shoe except for the nubs/ re-enforced bottom for the ATRs. Though I will say the ATR 3s and the Clifton 3s do fit a bit different for me in the forefoot. I have no issue with my standard 9.5 in the Clifton 3, but same size in ATR3 there is much more squeeze, thus I sized up to a 10.0.
The Clifton 1 and ATR1 were the same shoe, save the nubs on the Challengers. I still have one Clifton 1 and ATR 1 I may save or Ebay. I’m more partial to the 3s now, even if they are 3-4 ounces heavier.