I've heard that the 45% IAS from Treachery is better in most situations than the 300% ED from Fortitude, specially when CB or CTC decrep is involved. Could anyone help me to calculate the math so I can decide wich one is better for the rest of my setup?
Honestly, I cannot see treachery being better in any scenario. It could be a decent stand-in until you have a fortitude, but my merc always killed more and survived better with fortitude. The extra defence from shiver armour and 1500 base defence alone more than make up for that bit of DR he gets from fade. And that's not even assuming you made it in an eth armour specifically for the merc. Honestly, if you merc has trouble surviving, give him a delirium hat to go with the foritude.
Against act bosses, I can sort of see the point with treachery, but I am hard pressed to think of any of my builds that would go with treachery over fortitude even for that. Because more often than not, I can fit crushing blow and fortitude on my merc if I really need his help vs the act boss.
I mean, seriously. Just look at the damage numbers with and without fortitude and then compare that to the fpa with and without treachery and you've got a pretty good idea of what's going on. My guess is that fortitude will at least double your damage while treachery is definetly not going to take you from 12 fpa to 6fpa or something like that.
It depends on what you want from your mercs. For general slow-paced playthroughs, I would imagine Fortitude often would be the best choice, as it should make your merc a much more capable killer on his own, and I imagine that would be helpful in such a scenario. For specific runners, I would imagine Treachery being the better choice most of the time when playing melee character, and Fortitude being the better choice when playing casters. I use Treachery on most of my runners' mercs, because what I want the merc for is for his Might aura to trigger, or Decrep from Reaper's, etc. The merc's primary role isn't to contribute to the killing (where Fortitude would be good), but to power up the main character (where the IAS is good).
Treachery is an amazing runeword considering the cheap runes, but if you can afford Fortitude I can't see many reasons not to use it. Say your Merc uses Andy's Visage and Reaper's Toll, both socketed with IAS jewels, he hits a 6frame attack. Treachery would make that 5.5. Nice, but not all that much compared to 300ED and huge defense.
It's also a nuisance that Fade doesn't always trigger if you do quick runs which can make the Merc quite fragile at times. With Fortitude you can rely on him taking care of himself much more.
If you're looking at merc DPS only, Treachery's IAS makes it a better option than the other mid-rune armours, but it's not enough to beat Fort's ED. That being said, if you're playing something like a Whirler the merc's DPS will be fairly insignificant compared to what you put out and Treachery is probably worth it to proc ctc Decrip in that case.
Well in my case, I have multiple Treacheries, and no Fortitude, so a fairly straight forward choice. In terms of cost, Treachery is bloody brilliant, but if you have a spare Fort, it will win most of the time.
I'm a big defender of Treachery, but also a Vanilla player, so I can't properly compare. But I think the two runewords are in completely different strata- Treachery is good bang-for-the-buck and feasible to make untwinked, whereas Fortitude is really top-level wealth and much stronger than Treachery. Huge ED is not something that comes easily to a merc and though IAS is good for mercs that are used as appliers of elemental damage (enchanted mercs,for example) or good CTC (amp, and of course decrep) it seems like Fortitude is much stronger. The curse length reduction and DR are nice on Treachery, and there is the option often of the character and the merc swiching armors to keep Fade active on both, but still Fortitude is simply very powerful.
As far as the specifics in terms of damage per second, it's not so easy to work out as it depends upon the merc's other sources of gear, the auras present, and the merc's weapon. And the question of survivability of the merc depends upon how much leech, resists and DR% the merc has, as well as what kind of monsters are faced and what curses (for example, Life Tap) are likely to be around as well. It also depends upon how long the runs are- the Venom and Fade on Treachery may not even trigger during a Pindle or Trav run, for example. And Treachery can be used at a much lower level and is so much less expensive that having a closet full of various Treacheries is quite reasonable.