I can't speak for spotify, but last time I tried with google music the player still used a lot of bandwidth for whatever reason when playing only cached songs. There were also bugs like pressing next twice in quick succession having non-deterministic behavior, possibly due to network requests.

> Manually transferring songs like I used to with an iPod is something I don't want to have to go back to.

A minor price to pay for not being tethered to an inferior proprietary music player.