Ok, If I were able to play both, I'd probably prefer NightSky, because the physics puzzles seem very, very interesting and I have built lots of hype since 2007. Also, like LPChip, I generally prefer Nifflas' ball games than traditional platformers, for instance, I still prefer WaDF (and maybe even #mas2) to the games in the Knytt saga. Moreover, Nifflas' nightly environments are, in my opinion, the most interesting ones he has created.
Even so, there are lots of things to like about Saira, both in terms of presentation and gameplay and I'm sure you will not be disappointed.