It seems like Spotify has an early lead in this category. Like Amazon's Kindle service, it plays neutral third party in the mobile OS wars.
Google Music vs Spotify: quality
Currently, Google Music outputs at a maximum of 320kbps. This is quite good for the earbud crowd, but audiophiles who shell out thousands on their home stereo setups aren't getting their money's worth.
Same goes for Spotify, which also plays at a quality of up to 320kbps. Both services let you use your phone's equaliser to tweak settings.
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Obviously, high quality sound output over a streaming connection is difficult, and also threatens to eat up your data plan. However, the option to download tracks for offline listening gives the opportunity to increase quality substantially. It'll be interesting to see if either service chooses to one-up the other in this category.
Google Music vs Spotify: offline play
Google Music enables subscribers to download - or "keep," as they put it – up to 20,000 songs for offline play. It's well done on the Play Music smartphone app, where you simply "pin" music to the device through a menu tab, but is more complex on desktop.
Google only enables you to download each track twice through the web-based player. If you want to get more copies of your tunes, it requires you to download and install the Music Manager tool on your desktop.
Spotify, meanwhile, lets premium subscribers download tracks up to 3,333 across three registered devices. Both options allow you to enjoy music without having to rely on network quality, or eat up your mobile data plans.
One feature unique to Google's Music scheme is the ability to upload tracks you already own (or have stolen and hoarded), thanks to its desktop uploader utility.
This lets you save up to 20,000 existing tracks to your cloud account, meaning any gaps in Google's catalogue ought to be covered by your existing collection. It's nice having your personal collection right there alongside the streaming stuff, safely duplicated in the cloud for posterity.
Also, music purchased through Google's servers don't count toward this 20,000 limit, so if you pay for stuff there's no limit at all to how many tunes you can technically "own" on the site.
Google Music vs Spotify: social
Surprisingly, Google Music lacks social integration, at least to the degree that Spotify does it. It is possible to share an album to Google Plus, but you can't follow your friends and see what they're listening to. Nor can you ping URLs out to others. So no Facebook or Twitter support here.
Spotify works hand in hand with Facebook, enabling you to follow other users to see what they're listening to. You can also have it broadcast your current tracks on Facebook, plus Last.fm scrobbling lets you ping your tracks over to yet another internet radio system, if you still use that last-generation streaming service.
Plus every track can be shared from Spotify via the browser player, with URLs copied to your clipboard for posting to social sites or emailing to anyone. It's vastly more sociable than Google's surprisingly locked-down system.
Google Music vs Spotify: which one's best?
There are two things to consider, really - Spotify can be accessed for free through the web whereas Google's All Access service can't. But in Google's favour, those signing up to the UK servers in the first month get access for a promotional price of £7.99 a month, lower than Spotify's standard £9.99 ad-free access fee.
Spotify's been doing the streaming business and refining its service for many years now, and where it pulls ahead of Google's radio streamer is in terms of the ability to share tunes.
It lets you break out of the "walled garden" and share web URLs to tracks on sites and blogs, giving it much more of a social feel than Google's rather forced reliance on Google+ for sharing songs with mates.
The Google Play Music app is fantastic, though, and offers a way of managing your existing MP3 collection alongside streaming tracks and DIY radio stations, with one Google sign-in letting you access accounts and keep playlists synced across your various phones, tablets and desktops (up to 10 separate devices) with ease.
The understandable lack of an iOS Play Music app means iPhone users can pretty much forget about Google's radio ambitions, though.