There are a few aspects to consider including storage, data usage, audio quality, and accessibility. Though it takes much storage to download Spotify songs for offline listening, you can instead save a lot of data usage especially when you're outdoors.
Also, despite that streaming and offline listening can both get 320kbps lossy AAC audio, higher audio quality consumes more data usage. As for accessibility, you'll have a terrible streaming experience without or with a dodgy internet connection. As long as storage is not a concern, it must be a better choice to download Spotify songs for offline playback.