Grooveshark is one website that I'll stick to even though Imeem seems to be getting more users now and then. Reason being
1) Grooveshark's got a more simplistic, easy to navigate interface
2) The sound quality on Grooveshark is generally good (as you have the option to report certain songs that have bad quality, thereby ensuring consistency and quality)
3) Unlike Imeem, where you're bound by a 30 second restriction on some files, Grooveshark doesn't have that limitation.
There are stream rippers available on the internet (which will be in another review) and you could use them for both websites, but if you've heard a few songs on Grooveshark and Imeem, you'll know which is the clear winner.
As far as the interface is concerned, I took a screencap of the two sites as of this blog post. See which is more streamlined