I like Eagle Industries Tactical Assault Sling (TAS-4FX). I have one on my 870, which is also equipped with a Speedfeed stock. I didn't want to install a sling swivel in my Speedfeed buttstock because I felt it would be unable to withstand the load placed on it and pull out. The Eagle Industries sling attaches to the buttstock with velco loops, and costs about $23.00.
My 870 is also equipped with a Scattergun Tech side mount forward sling mount on the magazine tube. The Eagle Industries sling meets my needs very well and I'm quite happy with it.