I haven't noticed any improvements in Spotify's sound. Currently running Tidal and Spotify. Too many features on Spotify to give it up, even with the lower quality files. The playlist sharing, music suggestion algorithms, free hulu, etc. I would say I find new music from Spotify more than anywhere else. I will take music I find on Spotify and pull it up on Tidal for the main system.
Also, still no Metallica on Tidal. Not that Metallica needs hi-rez because it's mostly poor recordings and very compressed because they like that sound. But it's still something I want to regularly stream. I'm sure I'll find other artists over time, but I was surprised that one was still an omitted from Tidal.