I use both the NS and the WH and as has been mentioned, it's hard to say which one is better. The Warp Hunter has an unmatched potential for destruction while the Nightspinner is more for the 'trickery' kind of playing.
For playing against horde armies, i would probably pick the Spinner. It's high strenght and rending make it a viable choice, even against Space Marines. Personally i don't understand why people still use Prisms, they are clearly inferior when compared to the NS, unless you take two (and even then....).
I use the Warp Hunter in my Hybrid army, where it acts as an additional bullet magnet for the mech-wing. My opponents can either chose to fire at my Serpents, loaded with Banshees/Dragons/Avengers or the WH.
I have not read the IA11 yet, but if they didn't fundamentally change the rules for the Hunter, it's Flame Template still pretty much obliterates anything it touches at 2+. It can't get better than that
The Warp Hunter is more impressive overall and probably better at killing popular armies.