Both have their pros and cons. Audionetwork have a HUGE library of music from artists around the world. If you plan on using their music for personal projects, expect to pay $3 per song, which includes the right to post on YouTube or other video sites. If you plan on using commercially (like we do), the price will be around $200-$300 per song.
Premiumbeat costs about $60 per song, but this gives you access to both commercial and personal usage. What is cool about their system is that you can buy songs as well as the respective musical loops that can help you create the song you want. If you enjoy music editing, this is a cool feature.
We have tried other sites like Envato, but found the music quality to be on average much lower. There are some great talented musicians selling on Envato, but right next to them are low quality productions which downplays the quality of the site.