Sorry if i have misunderstood but you want to stream from your ipad? Do you mean you want to listen on your ipad?
Unlerss its changed recently spotify didnt have an app for the Ipad (only the iphone, which will run in that horrible small window on your ipad).
I used to (and still do) love spotify but as a sonos user I have moved to napster because of a year old problem with lots of tracks not being available in spotify when accesed via sonos.
I bought an ipad2 recently and must say the napster app looks great (as does the sonos one).
If you work around the facebook hinderance and you dont have sonos and dont mind waiting for the ipad version of spotify and want better quality sound then go with spotify.
If you have sonos and how good it looks now on your ipad is important then go with napster.
I recently moved from spotify to napster.
Oh and the tenner a month is great (from both providers) rather than the £5 a month offering.