I haven't used Pandora in a decade, but when I did, it strongly preferred to play mainstream artists, and within artists, only a handful of specific tracks. They weren't bad choices, but I got pretty bored of them.
Spotify takes a lot more risks and I think its highly preferable.
I would believe they've improved over time. But my comment wasn't that pandora had a poor catalog. It was that they wanted to play mainstream songs instead. Perhaps not even mainstream, but just very specific songs. They had a lot of artist's whole discographies but only a subset would ever come on. I can still remember a few of them for me:
royksopp - this must be it
trapt - headstrong
ladytron - destroy everything you touch
Some sort of remix of the hand that feeds by Nine Inch Nails (I think it was the photek remix or something)
I got these songs a lot on many stations, and nothing else by those artists ever, haha.
Spotify takes a lot more risks and I think its highly preferable.