They are extremely similar. I've had the free version of both for years. I recently switch to premium version because both seem to have increased the frequency of adds on the free versions. You're correct that the adds aren't long, and I also grew up with broadcast radio/television so you learn to deal with adds. However, it was starting to seem like you got 2 songs and then an add. Not bad if they're songs you like, but with the free versions you got limited skips, and you'd end up prompting another add quicker.Seems Pandora and Spotify are pretty similar. What are the differences? It seems more people use Spotify. I have a few different Pandora channels I listen to at workz or when working in yard/garage, but I just use the free version. One is a default channel on Pandora called Best of the 80s reloaded, and is better than other 80s mixes I've listened to on other platforms(IMO), one is a channel built off a single song I liked, which I haven't actually heard that song listening to the channel is quite a while. And one is built around an artist I like, but it actually plays a good variety of other songs, even different genres. It's also my most listened to channel right now. The ads don't really bother me, but I grew up listening to broadcast radio, and they aren't that long when they are played. In my truck, I have SiriusXM subscription.
Anyway, back to your question. The main difference is how they're set up. They both have the same features but they're laid out differently. Pandora seems to be set up more for "station" listening, whereas Spotify seems to be set up more for listening to songs on your library. Both work either way though so no real difference other the set up.