I'd probably go for Cardiac, simply because it's the only way to significantly boost your resistances, at this point - the endurance fix is a bonus.
(It would also let you drop Energy Mastery for either Pyre for damage, or, going red-side for a bit, Soul for damage *and* Darkest Night.)
Spiritual... well, it'll enhance your heals and maximum health, which is nice, but the recharge bonus is wasted on Willpower.
Or you could wait until the new alphas go live. One of them boosts resistance,and there are others boosting healing and defense as well.
I have no intention of filling every incarnate slot on every toon, but I would like to at least fill each of my 50s' Alpha slot, at least to Tier 3 to get the Level Shift since that is at least useful outside of iTrials.
So if you were going to fill one and only one Alpha slot on a WP/EneMelee, which way would you go? I think Spiritual Core but not sure.
Since the toon is no where near softcap, I'm also considering Nerve. Willpower and Energy Mastery means Cardiac isn't really necessary. And Musculature only helps one thing (damage), so not sure that's critical either.
Here's some of the toon's numbers. (With all toggles on, but SoW off)
S/L Def: 33.06%
F/C Def: 37.4%
E/N Def: 38.02%
Psi Def: 34.64%
S/L Res: 65.56%
F/C Res: 16.61%
E/N Res: 11.57%
Psi Res: 40.36%
Tox Res: 21.57%
2925.89 HP's
1.94%/sec Regen rate (56.71 HP/sec)
3.97%/sec Recov rate (4.44 End/sec)
1.38%/sec End consumption (1.54 End/sec)
1.66x Accuracy bonus
4% Damage bonus
"Everybody wants to change the world, but nobody wants to change themselves." -Tolstoy