One advantage of the HD7 is that, being a first-gen phone, it's highly hackable. If you want to do custom ROMs or heavy tweaking of stock ROMs, the HD7 is your best bet. It also has the largest screen, which help with things like using the on-screen keyboard.
The Radar strikes me as the Trophy v2, a device I was never terribly impressed with to start. However, it does have the advantages of a second-gen device: the faster processor and the front-facing camera. The CPU quality is not yet an issue, really - some games might load a little faster, but all of them are designed to play very smoothly on first-gen phones - but it might be a bit more feature-proof. It's also worth considering that pretty much any phone can be "skinned" with coatings that make it more resistant to damage and change the color, but are cheap and add little weight or size.
The Lumia 710 is a very nice device, but I agree tha 8GB is just too little storage. Is it possible to tear it down and put in a bigger microSD card? Many phones, including the HD7, don't actually have a soldered-in storage chip, they just use an internal uSD that can be replaced if you know how.
Google Music could be done on the phone, but it would have to be via an app. A quick check of the marketplace turns up nothing useful. Hmm, maybe this should be fixed... You can also use Pandora (and any other site that supports HTML5 streaming) on the phone; just open the page in a browser, and the music stream will start playing (you can control the station via the browser windows, of course) and the play/pause/volume controls that are built into the phone will be usable from any app, no need to keep the website in the foreground.
However, WP7 natively supports Zune Pass. For $10/month (less if you buy a multi-month subscription), you get all-you-can-eat music streaming. It can only be played on Windows Phones, Zune software on PC, Zune devices, and Xbox 360 (the files are DRMed, and will also stop working if you stop paying) but the selection is very nice, and my HD7 makes a good portable music player. It's a great way to get around the issue of not being able to keep your whole music collection on the phone. It's also a good way to discover new music; there's a feature called "Smart DJ" that automatically creates playlists based on songs, albums, or artists.