This is what we have been using for a while. Seems to get a little deeper than cable (less drag) with no hum and no fraying ends to catch your fingers! All we used was a doubled line uni knot and it has held up well. We used 200lb on our downriggers.
American Fishing Wire RB200GR-450FT
Downrigger Braid
AFW's
Downrigger Braid offers high strength with minimum diameter and is 100% Spectra Braid. AFW's Downrigger Braid is tough and abrasion resistant. This Downrigger Braid will also help eliminate trolling hum.
American Fishing Wire RB200GR-450FT Downrigger Braid