There really isn't a definitive answer. Both of the set bonuses are good. On one hand, you have a short CD survival buff (PW:S granting 1200ish resilence) vs. dispel protection for your DoTs. Personally, I'm going to go with the Shadow (Satin) set myself because I feel like our survivability is decent with Glyph of Dark Binding and not having to drop shadow form for most of our heals. It also depends on what kind of comps you plan on running in arena. If you plan on running with a Warlock, for example, you already have dispel protection.
Honestly, it's like like our DoTs do so much damage that it really hurts when they're dispelled in PvP. No, the thing that makes it sting when our DoTs get dispelled is the procs you get if you're spec'd into FDCL and DI. DoTs are crucial to those specs, but not because of their damage but because of their innate procs with that spec. That being said keep in mind the 3 second fear has NO diminishing returns and is AOE within a certain range. Yeah, it's not quite as nice as the dispel protection Warlocks have but it's still nice and better than a slight increase in damage reduction when I pop PW:S, at least imo.
The fact that dispel removes everything now is such a buff to the VT dispel horror. You VT and then fear the DPS and the healer will get CC'd for 3 seconds every time. No more lucky dispels where the healer gets the fear instead of VT.