Recharge, definitely. In a farm, on a tanker, you are likely to be damage capped all the time with red inspirations, so Musculature does nothing, zero, zilch, nada for you.
Even for someone who doesn't use inspirations, I agree with your analysis. On a Fire/SS one is already going to run between 280% and 360% damage (1 or 2 rages). Adding an additional 32.5% (net boost from T4 musc after ED on ED capped powers) damage isn't going to do much.
Fire/Fire is a bit different as it doesn't have damage boosts from the secondary save for BU, which isn't up most of the time. I could see Musc possibly providing a bigger bonus than Spirit here, on a high recharge build.
But, again, let's stress that this is all without using inspirations - which is kind of shooting yourself in the foot generally, and right in the head when it comes to farming. 
Thanks, Nihilii, that's basically what I had been thinking...with all of the red insps and other damage boosts, musculature just seemed meh to me, but I thought maybe I was missing something since they seemed so sure of themselves. And truth be told, I'm not that familiar with how the incarnate system works (just a basic understanding of how it works with the ED cap) so I figured I could very easily be wrong.
Musculature doesn't seem to pack as much punch as any of the other Alpha abilities for most* hero/villain performance. However, I see that changing once the new Incarnate abilities hit the live servers. As of right now, I can honestly think of very few situations in which a tank wouldn't benefit more from either more recharge/healing (Spiritual) or endurance/resistance (Cardiac).
*Unless you have some weird build in which your attacks are slotted for Taunt sets and need the damage. But then, why would you build your tank like that in the first place?
I'm curious what people's thoughts are on this, since I got into a discussion in a group tonight about it and they have me rethinking my opinion on it.
I thought the recharge incarnate (forgive me, I don't know the name offhand) would benefit a pure farming build the most. Since it passively adds damage through more attacks, and through more uptime on build up/rage/fiery embrace.
Two people in the group swore up and down that musculature was the way to go for a tanker, but I had always thought that that was overall one of the worst for tanker (maybe not for farming though).
Anyway, the only influence/infamy farming I do is on my brute, where it seems like recharge is king.
Would musculature be a good choice for a Fire farming tanker (no, I don't plan on making one)?
Sorry if this has been discussed before...I did a search but couldn't find it.