Is there an advantage of using one or the other (aside from cost)?
I'm about to add trim to a tarp (among other projects), and I expect to go with grosgrain. But I've also seen bias tape at the local fabric stores, and even online in the same section as grosgrain. The bias tape at Joann's looks like it's for quilts as opposed to outdoor gear, but at OWF or Quest it comes in nylon and appears to be designed for a double-fold trim (sew one free edge of the tape, fold it over to sandwich the fabric edge, sew it shut).
Most of the threads I've seen people are using grosgrain, with a variety of methods for attaching.
EDIT: I am familiar with this post http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/45819/index.html, but i was hoping to get some more insight from the BPL faithful.