I tried out both a couple years ago, found them to be pretty comparable in terms of music catalog, and chose Rdio because I prefer their interface.

Recently, I've been noticing that some of the "people" I follow have stopped publishing playlists to Rdio in favor of Spotify, so I've been considering the switch. But it's a shame to have to leave Rdio's great HTML5 web interface for either Spotify's native app or flash.

Yep, completely agree. I think Spotify had a really good opportunity to innovate as a streaming music player but instead they created a Frankenstein's itunes. I'm not mad that they copied an existing interface but that they copied one that was terrible to begin with. I really hope that the work Rdio did in terms of their UI doesn't go to waste.