Incarnate slotting can vary from person to person and from playstyle to playstyle. There is no hard and fast rule that says.. You have this set... you take this.. Its amore about what YOU want out of that slot. I would probably look at Musculature which would push your damage over the top for all your pets and yourself, Spiritual which would give you faster recharge for Lightning Storm possibly getting 2-3 clouds out at the same time, or Cardiac if you have end problems..
The hard things I can do--- The impossible just take a little bit longer.
If numbers are so much more important than a teammate who is fun to play with, forget about the game altogether and go play with a calculator instead. -Claws and Effect-
Yeah if you really can't decide the default is always Musculature, more damage is always a nice thing.
I actually find /storm a very endurance heavy set especially I carry 3 leaderships. I went with Cardiac.
If I remember correctly, Demons use quite a bit of endurance too so they'll benefit from Cardiac as well.
My basic rule for Mastermind Alpha is this: Do I need more damage or more endurance discount. The rest is just gravy.
What's left is to normalize all Assassin Strikes and improve Stalker's old sets (Claw, MA and EM)! You don't need to bring back the missing PbAoE attack. You just need to make the existing ones better! For example, make Slice a WIDER and LONGER cone.
I did Musculature for my bots/FF character, and Spiritual for my bots/traps, and like both results. I'm thinking about making a Musculature for the latter, to compare.
But really, I suspect anything you have fun with will work. 
Masterminds arrive with fewer outright needs and holes than many archetypes.
Thanks for all the replies. Looks like damage it shall be since I have very little problem with endurance and my recharge is fairly high allowing multiple stacks of LC.
Doc M
Doc Midknight Dark/Dark/Dark Defender
Nether Raven Dark/Dark/Dark Scrapper
Master Midknight Dark/Dark/Soul Troller
Global: @DocMidknight (main) Global: @Doc Midknight (alt)
I have recently gotten my first mm to 50, he's a demon/storm and I was wondering which alpha slot should I unlock for him.
Thanks for any help on this
Doc M
Doc Midknight Dark/Dark/Dark Defender
Nether Raven Dark/Dark/Dark Scrapper
Master Midknight Dark/Dark/Soul Troller
Global: @DocMidknight (main) Global: @Doc Midknight (alt)