There are some differences of course. Mahō Shōjo Neko Taruto is an older full-length show from 2001, and Nyanko Days is a 2-minute short show from 2017. But the one huge similarity is that it's all about the cats, who are portrayed as small cute catgirls.
Both show cats as small people with cat ears and tails, and each has supporting character who is a cat maniac/collector who trys to capture cats in a butterfly like net. All that aside tough Cottonland dose has somewhat of a darker undertone to it
Kaishaku created anime; displays cameos of their animes: Kannazuki no Miko, UFO Princess Valkyrie, Magical Nyan Nyan Taruto, Steel Angel Kurumi.
In the Magical Nyan Nyan Taruto prospect, it displays an alternate character called Tarurotte, who resembles Taruto's character and has the same Japanese seiyuu.