I used all 3 before. I used to use Pandora, but I realize Pandora has a tiny library. Something like 600-700k songs only. iTunes had that kind of library size like in 2004 or so. Slacker, on the other hand seems to have a MUCH larger library. I feel like Pandora plays top songs I know whereas Slacker takes me to newer songs.
In terms of audio quality I notice Last.fm and Pandora lack bass. Slacker seems to offer more accurate sound but at the same time I believe they use like 40kbps AAC or something so you can hear the noise in the audio. Last.fm is pretty nice in terms of quality but once again its very lacking in bass. Make sure you turn on high quality audio on Last.fm. They claim its 128kbps but somehow they compressed the bass and lost it. Not sure waht kind of compression Pandora and Last.fm use but it's suspicious that the bass completely drops out. Low frequency sounds should usually be pretty well kept. Youtube had bass tests back when they used to have ugly 320x240 video and terrible audio. Yet you could still run bass tests and subwoofer tests using certain Youtube videos. Anyway, I'm no expert.
I hate how Slacker pauses a lot though. Either it has a terrible caching method or iono. The ads are a little annoying too. However the app interface is beautiful. I feel like Pandora took like 5 minutes to code. They obviously made it very bare bones compared to the web player.
What do you guys think of these 3?
Oh also, please try to keep the desktop versions out of this. I know many people used Pandora on their desktop and Slacker is less used. They're different on their desktop platforms as I know Pandora plays a lot more ads on the desktop version than on the Mobile versions. Plus, bitrates are probably different as some mobile versions like Slacker specifically use a lower quality stream for mobile networks to save bandwidth