I've had very few buffering issues with Spotify via LTE over the years when compared the constant dropouts from my SiriusXM in my various cars that have had this service. SiriusXM audio quality is also terribly compressed and tinny in addition to the constant drop outs. I've never had a good experience with their product.
Spotify can also download large quantities of music and playlists...so with minimal planing you will have no buffering issues.
Hard with a 16 (actually 14) GB iPhone. I deleted my iOS music library (to free up space) and only stream now. Even with the LTE issues, it's (streaming) better than listening to the same couple dozen songs and dealing with iTunes.
Spotify can also download large quantities of music and playlists...so with minimal planing you will have no buffering issues.