Try them all. They don't cost anything that you can't farm for until you get past the 2nd tier. It isn't like a build where you are locked into one choice until you get a respec.
Musculature will likely disappoint you. With Soul Drain saturated, Musculature will barely give you a 13% increase in damage at tier 4. Spiritual is the most popular choice. It will increase your healing from Siphon Life as well as make it cheaper to reach perma-Hasten.
Without seeing a build, there is little more advice anyone can give you.
My build is pretty much a joke. To be honest, I was never a fan of the combo, but I got it to 50 just for the sake of getting to 50. The whole "pure single target" thing doesn't really appeal to me. So I'm not going to spend too much time on this guy. I just want to get an alpha slotted so I can do stuff like the Apex without the penalty.
The build is simple. Just SO's everywhere, except 3 attacks, which are slotted with sets that get me to the cap with the SR powers + weave + steadfast unique.
Do you think spiritual would still be better than musc even with an essentially SO'd build?
If you're on SOs, I think Spiritual would be arguably even better than it would be on an IOed out build. There's little chance you're already ED capped for recharge on most of your powers, so you're going to get a full 33% boost on many powers from just the common one.
Hmm ok. I went spiritual on my fire/SS tank, and I like it a lot. Guess I will do the same thing on this guy. Thanks guys.
I'm late to the party, but yes, Spiritual. You can use your better attacks and Soul Drain more frequently to do more damage. You can use Dark Consumption more often to take care of any endurance problems. You can spam Siphon Life faster if you're in trouble, plus it will heal for more hit points. And yes, this applies even to an SO build, and perhaps particularly to an SO build.
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My i1 DM/SR is in the same boat. He's decked out to the 9's with all the yummies- even using PvP IO's from long time of marketeering. I went Spirit, and have T2 musc. I really dont think any of the Alphas are all that great for it, and with DM/Shield with T4 spirit (and hami-o's for DDR), I hardly play him anyway.
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Well, I just need 2 more shards for my tier 1 spiritual. 
For awhile I had T2 for Musc and Spiritual, and I flipped back and forth. I ended up going on to T4 Musc, mostly for the boost in End Mod and Damage. The build is roughly 6 seconds short of PermaHasten anyway, and I tend to get by quite nicely with where my heal is at.
The build as it stands on live is roughly unstoppable. Running Incarnate TFs is a breeze. There is heavy head use with a constant use of FA, but I have a recovery of over 4 end per sec, so on the rare shots I actually run below half a tank, using Dark Consump fills me back up.
tl;dr I have had the best luck with Musc. Althought seeing how the alpha slot salvage is useless for anything else, I think I will work towards T4 Spiritual for kicks.
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As recently as a couple days ago I was 100% sure that I would be going for the cardiac, since the biggest problem this toon had was ALWAYS endurance. For many years.
But after improving my build a bit this weekend, I've found that I've basically gotten rid of my end problems. With phys perfection and the 2 +rec uniques, it's hard for me to run low on end. And if I do, there's CP and dark consumption.
I don't know what the hell I did, but the change was insane. On paper it didn't seem major, but in reality it's like a totally different toon. So all of a sudden, something that was completely obvious is no longer obvious. In fact, I'm pretty sure going cardiac would be a total mistake.
Now I am leaning towards musculature. One of the paragons improves a bunch of effects, and almost every single one would apply to this build. Any comments or opinions about musculature for DM/SR? Is it a good choice?