I have subscriptions to Nokia Music+ and Slacker Premium ($9.99/mo.). I used to have Slacker Radio Plus, but upgraded to Premium a few months ago because I like their service so much. Anyway, for $3.99 per month, Nokia Music+ and Slacker Radio Plus are pretty comparable. They both offer an ad-free experience, unlimited song skips, lyrics, and the ability to download stations for offline listening. Both services also have apps for Windows 8 and RT as well as web sites so you can access the music on multiple devices. Also the mixes available on both services are curated by real people (DJ's, Musicologists, etc.) as opposed to computer algorithms like some other services.
So what's different about them? Here's what I've found. Nokia Music+ lets you create custom stations by choosing up to three artists. With Slacker, I believe you need the Premium subscription to create custom mixes. However, Slacker allows you to ban or favorite songs. If they play a song you absolutely hate and never want to hear again, you can ban it. Hear a song you love that you want to hear more often? Just favorite it. This is a feature I wish Nokia would add to their service. As far as audio quality goes, I'm no audiophile, so both services sound very good to me; though I believe Nokia Music+ streams higher quality audio than Slacker, but I'm not sure. If you store music on your phone, then Nokia Music+ gives you access to it (like Xbox Music), but the current Slacker app does not. Speaking of the Windows Phone apps themselves, right now I think Nokia Music+ has the nicer app; however, I've gotten confirmation from Slacker that they are currently working on an update for their Windows Phone app to coincide with their new look and features. I'm eager to see what the new app will offer. As to the music mixes offered on Nokia and Slacker, I personally prefer Slacker's mixes over Nokia's for the music I listen to (mainly Alternative, Indie, Pop, and Classical).
For the $3.99/mo. subscription, I don't think you can go wrong with either service. They're both really good. It may boil down to whose music mixes you prefer. Now if you're willing to shell out $9.99/mo., I think Slacker Premium offers a lot of great features. You can play individual songs or albums on demand (I don't think you can do this on the phone app yet). You can create single artist stations, so if you want a station that plays only your favorite band, you can do that. You can create custom stations by specifying as many artists as you want. You can then fine tune that station to play only the artists you specified, or to include similar artists. You can further fine tune your stations specifying whether you want them to play more or fewer or your favorite songs, more or fewer popular-on-Slacker songs, or newer or older songs. Another cool feature is that Slacker automatically creates a station of all the songs you marked as favorites. For now you have to create all these custom stations on Slacker's web site, but once they're saved to your account, you'll have access to them on the phone app and can cache them. You can also create your own playlists.
It'll be interesting to compare the services once Slacker releases their new app. I personally love Slacker Premium and think it's worth the price. Once they update their app, I think it will become one of the best music services on Windows Phone. For the $3.99/mo. subscriptions though, it's tough to decide between Nokia Music+ and Slacker Radio Plus. I think I like Nokia's phone app better (for now), and the ability to create custom stations by seeding up to 3 artists; however, I prefer Slacker's mixes. They just seem to play more of the music I like. I find myself skipping songs more often with Nokia Music. I rarely skip a song on Slacker. Anyway, both services have features I like, so I use both for now.
I hope this helps you somewhat. Ultimately, I think you just need to listen to both services and see whose mixes you prefer. If you're happy with Nokia's mixes, like seeding your own mixes, don't care about banning or favoriting songs, want access to the music stored on your phone all in the same app, and don't want to spend more than $3.99/mo., then go with Nokia Music+. However, if you like Slacker's mixes better and think you may want the Premium subscription, then go with Slacker. Hopefully, their new app will be available soon.