How are you with accuracy?
I have Spiritual on my WP/SS tank, and I enjoy it a lot.
I have plans to slot my Beam Rifle/Mental blaster with Musc when he gets Alpha.
There's also Agility, which adds to End Mod, and movement rates at the upper tiers.
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In general Spiritual and Cardiac will give you the most benefit since powers generally aren't ED capped for endurance and recharge. Musculature is nice for Blasters too, since even if you are ED capped for damage (which you probably will be) you'll get another 30-35% or so damage enhancement from the tier 4 core ability which will give you something like 15% more total damage. Nerve is possibly useful on some builds but in general I find that I'm either hitting something 95% of the time or I'm missing too often for some extra accuracy enhancement to help much (high defense enemies, massive to-hit debuffs). If you want to fight higher level enemies and tend to be in the 80-90% range on to-hit against them then Nerve will be handy, but otherwise not so much.
For a Psi/Mental Blaster I'd lean towards Spiritual if you want more recharge for a higher Drain Psyche uptime or Musculature for more damage. The new Vigor alpha might also be an option... it boosts healing (more regen from Drain Psyche), accuracy, endurance, and if you use World of Confusion (for something other than a set mule 
) the radial branch of Vigor also boosts confuse. My personal choice would be to go for the Spiritual Core line, since that gives you maximum recharge plus some healing enhancement. More Drain Psyche + more regen from Drain Psyche is a good thing, and recharge never hurts on your attacks either.
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If you can't get perma-DP, go Spiritual. If you already have perma-DP and high defense without dipping into the alpha slot, go Musculature (the effect on your attacks is small, but it boosts your judgement and lore pet damage too). You should not need Cardiac with drain psyche giving you limitless end.
Ive just started playing again and just unlocked the Alpha slot for my Psy/Mental blaster. I am just about coming to grips with the whole incarnate system and had a few questions.
Basically, what is thought of as the best Alpha slot? I was thinking either Spiritual (for recharge) or Musc (for damage). Current damage bonus is 33.5% and recharge is 120% (with hasten). I also dont really have that many end issues.
Any help appreciated, thanks.