Do you know if QSpot allows for tracks to be saved to device for offline listening? Spotify Premium is the same price as Google Music so I don't mind switching. I only have 1GB of 3G a month though, which is why saving to the device is such a big thing for me, I download it all on the WIFI at home.
The truth is, Google services and Android apps are going to work much better on an Android device than on an N900 (if they work at all), so if you're happy with those services, it'd be best to hang on to your One...