Its hard to analyse this quantitatively without bias since there are no indicators of quality in the Spotify App while a song is playing.
As you say the quality thing will only matter to some people. The ability to store music offline is also becoming a less important feature as mobile networks improve. I have to admit I still use it quite often but looking ahead a few years Spotify really needs to differentiate their premium product more.
I only need it when I don't have wifi, but I think I'm going to start using mp3 players again because managing the whole download/undownload thing in Android Spotify is tedious and the overall useability of the app is crap.
The app itself is sometimes a pain (particularly via the controls on my car steering wheel...), like the inability to change whether I've liked or disliked a song.
I have a 50MB limit. And the stupid app doesn't let me differentiate between playing offline and playing only when I have wifi, so if wifi is turned on on my mobile and also the 3G (which I might have on because I want to browse some websites when I'm outside my home or going somewhere), and the wifi doesn't work for some reason, Spotify will happily leech off my data plan with me being none the wiser.
£15 a month, sim only, on 3. Also a £12.90 option, but with less inclusive texts and minutes.
What the mid-tier plan gave us was ad-free listening. It sucks that you know have to pay twice as much for that.