The defender damage cap is 300%, and a little under 100 of that is supplied by damage enhancements, so you don't need to worry about the damage cap until you're in the neighborhood of 200% damage buffing. I'd say as long as you're able to reliably hit +4 enemies the musculature is probably your best bet.
I love my D3 as well, and the only thing I wish he could do better in a team from the trip from 1 to 50 was more damage. He has a great enough heal/debuff set/team support, just gotta make those DoTs really hurt and he's pretty much my all-in-one toon. 
On my dark/ice I am leaning to spiritual for first, recharge, and added stun in howling twilight. My ice blast does blaster damage, and more damage is appealing, but so is faster recharge on ice storm, blizzard, and power sink.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
For my namesake, my D3, I've opted for the Cardiac route. Specifically the Endurance/Range side.
It's meant that I've endurance enough to ditch Dark Consumption from the APP's, and pick up Hasten and Soul Drain after fitness went inherent.
My damage is a lot more than it used to be, with Assault and Soul Drain, I'm usually running about with 150% or so damage buff. With the faster recharge on top, I'm far more violent than I used to be.
Also most of my damaging attacks are at 95% or so from IO's, so the Musculature Boost won't really do much for me. But with most powers having 30-60% Endurance Reduction, the majority of the Cardiac Boost is working.
I, personally, would rather use the whole of a boost, instead of 1/3rd post-ED. I'll also be sticking with Cardiac for the Very Rares, as I'll be going the Fear route, so I can swap in Accurate To-Hit Debuffs in Fearsome Stare, and still have a decent duration. (Keeps my 6.25% Recharge bonus, but frees up another slot).
Hello fellow Defenders.
With i19 now live, I have returned to my first love, my D3 (Dark/Dark/Dark) Defender, my namesake and I'm having a wail of a time taking him through the Incarnate arc and teaming with fellow 50s on many TFs.
However, one thing is bugging me. If there's one thing that annoys me on Dante's build, it's his lack of damage. Now, I have built for damage increases and have 40% damage buff alone before adding on the solo boost that Defenders get. So at the moment, I am eyeing up a Musculature based Alpha slot.
However, if I add more damage to my build, will I eventually reach the AT cap? Would such a route be wasted? I already have decent recharge, end recovery and accuracy so damage seems a decent goal. I'm just wondering whether something else would benefit a D3 more?
Any thoughts?
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk