I went with Dark Mastery originally on my FF/Sonic and it was TOO good. Oppressive Gloom and Screech enabled me to keep bosses stunned as soon as I got into melee range with them, to the point where all my carefully assembled defence was meaningless 
I respeced into a mez-free build with Electrical Mastery, basically for the Energy Resistance.
Dark Mastery also opens up Soul Drain later on, to beef up your -Res with a bit of +Damage. The two are always good together since they multiply.
I'd say Dark Mastery for sure.
As for defence slotting:
Slotted Dispersion and Manouvers is 21.3%, A Steadfast IO in Dark Embrace (or whatever) is 3%, Hover or CJ is 2.5% unslotted for base defence of 27%. You're 18% short of the cap.
Ranged first: Red Fortune in Manouvers and Dispersion, plus three Thunderstrikes in your ST blasts gives you 16.25%, close enough. All sets need 6 pieces.
Screech takes Razzle Dazzle for AoE and Melee +2.5%
Oppressive Gloom can do this too. The chance to immobilise is nice, but this power really doesn't need slots, so you're using slots just for the set bonuses here.
Gaussian's Synchronised Fire Control in Tactics adds 2.5% Melee and AoE too (Thunderstrikes and Red Fortune will saturate your +2.5% Ranged def bonuses though)
Aegis (5 pieces) in Dark Embrace is +4.6% AoE
So, two razzle Dazzles, Gaussians and Aegis puts you at
34.5% vs Melee, 39% AoE.
I 6-slotted Stamina with Performance Shifter to get the endurance proc and set bonuses, another 3.13% AoE there. Thats 42% vs AoE.
You can always take Combat Jumping and Hover (both 2.5% vs all, and 4% when slotted for Defence.) This would cap you to Ranged and AoE, and leave melee less than a small luck pill away from capping. Melee mitigation is also dealt with via your stun aura and Hover if you take it.
Ok Veng is out.
Any build advice?
I am looking at slotting for defence.
Oh and what about epic pools? Which one would be better suited for a FF/Sonic?
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