I'd say to go with Soul Mastery, just because the armor has the best combination of resistances, but it depends on what you're going for and what your play style is. Thugs/Thermal is pretty flexible that way.
Cold Mastery would be pretty awesome too.
All my masterminds get Soul Mastery...
Just sayin'.
The combination of hold, immobilize, and cone-ranged attack fills the holes in every combination I've tried. And for me, the armor is icing on the cake.
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AE Story Arc #536752: Torn Asunder
An army from far, far away has been driven from their homeland and landed on Earth. They desperately need a new home and they're liking the look of ours.
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Well two votes for Soul Mastery! Fire makes more sense thematically but I have to say the Soul Mastery Shield probably beats the fire mastery Rez. (Espically for a mastermind... A self Rez on a mastermind?)
Plus you can NEVER have too many tentacles! Especially weird albino tentacles!
Personally, I don't feel the need to get an extra attack on my Thugs/Therm. I'm planning on going for Mace Mastery.
Mace Mastery gives Defense from Scorpion Shield, as well as a nice AoE Immob, a hold, a cone blast I have no intention of taking, and POWER BOOST.
Power Boost is your friend. Your bestest friend. The best friend of any therm. It makes your debuffs more devastating, your buffs more beefy, and makes healing far easier. I love that power and use it in conjunction with my buffs and debuffs on other characters with scary success.
On my DEMONS/Therm, though? I plan on getting either Soul or Mu.
Too many alts to list.
Trouble for Therm is that Power Boost doesn't enhance +Resist, so your Thermal Shields are unaffected. Also it won't enhance +Damage, so while the +ToHit aspect of Forge is improved, the +Damage aspect is not.
I'm not sure if the -Regen in Heat Exhaustion gets a boost or not. The -Recovery certainly does, for what that matters.
Melt Armor's Def Debuff is improved, but it's -Resistance is not.
http://paragonwiki.com/wiki/Mace_Mastery#Power_Boost
I have a thugs/thermal mastermind that just hit lvl 41. What Epic or Patron pool would you recommend and why?
Also I'm sad that Beast Mastery isn't available... but I'd rather they got it right then released it too soon!