Theres advantages and disadvantages to them both. I personally run filter socks (felt socks, not mesh) because they catch a ton of junk and are easy to clean. I do have to clean them every 4-5 days though, so some see that as a pain. Bioballs offer a ton of surface area for bb to grow on, however they have been known to cause nitrate build ups. Also they take up a good bit of space, space that could be better utilized by live rock rubble.