While Caldara does look better than Rugani at the moment, one should be careful.
Caldara is playing in the middle of a back 3, Conte and Bonucci taught us how many defensive mistakes that can hide. It sounds hilarious but Toloi and Masiello have been great this season, so Caldara has a solid platform to build on.
At the U21 this summer when they were the CB pair Caldara did look like the best one. Mostly it seemed due to confidence as he came from a season as a full starter while Rugani was on the bench most time and then injured for the end of it.
They are just very different, luckily we'll see which one is best here. Rugani is more Barzagli, Caldara is more Chiellini.