Zune is peerless: Its combination of online and offline features is not available elsewhere, at any price.
But, for you, it's a moot question.
You don't have a dilemma; your wife has a barrier. Zune is better in a number of ways, but she cannot use it on her phone, and given the limits you've set for yourself, you have to go with Spotify or another "premium" service.
You both can listen to a Spotify-Premium playlist (only), offline, but you have to toggle the spotify apps to offline mode, and then stay off. After you agree on a given playlist, you'll both be able to play it offline. Being in offline mode, neither of you will be able to also tap the online features that you're paying for.
I listen to my entire Zune Collection offline and can still tap the Marketplace for Smart DJing an artist, or for Bing-listen-searching-and-downloading tracks or albums, or doing any streaming. I go back and forth, dynamically and seamlessly.
Both services limit streaming to a single device/account/ID at any point in time.