It makes sense to list some budget / midvalue / best in slot items because it depends when in a ladder you make a build. It only takes a few weeks into ladder before most BiS setups are easily attainable, a Zod too early for obsession could be 5000+, but right now you can get a -15 mang song in the ~500 range and that's a perfect roll.
There are some +1/-1 skill and +%/-% fire and resist tradeoffs which are individually pretty minor, in mang song vs obsession, opalvein vs fcr ring vs soj, resist boots vs wraithstep, hellwarden's will vs flickering flame.
For the boots its essentially +1 skills vs 3x 40% resists and ~80 life in saved stats. And in ring slots, you can get significantly higher resists on FCR rings, up to a max of 2x 31% / 2x 11% resists, so the 5% fire skill damage on opalvein is weighed against those straight resist prefixes that are possible. Mang songs opens up an extra socket which means its +1 skills / +5% fire / -20% resist vs obsession's +35% fcr / 70% resist / 25% life and other minor stuff. Since magefist/arachs/crafted ammy are all BiS in every setup obsession hits 125% with no ring/helm/shield fcr needed if the ammy is 20%, 1x opalvein on a lesser roll. Mang Song needs 2x rings and either 20% circlet or hellwardens, but can still use ars baalos, and then any 15%+ crafted ammy.
The thing is the tradeoffs definitely favor a mang song's setup for maximum damage output. Even if you used 2x soj and wraithstep instead of any fcr/resists in rings/boots, you're up +2 skills but the mang songs setup is up +15% fire / -20% pierce from the 2x opalvein + mang songs + facet, and you've only got an extra ~10% resists and 25% max life / 50% max mana as advantages. Both builds can use a +3 skills helm, because a +3 demon / 20% fcr / 2x facet magic diadem is better than a hellwarden's will and gives the same +3 skills as flickering flame, with +10% / -10% vs -15%.
When you put it all together, I'd say the mang song's build hits a higher maximum damage and is a faster kill speed as a result, but the real cost starts coming when you want to max out your resist in those other slots. For example I have a +2 lock / 15% fcr / 20% all resists crafted ammy, that was not cheap. Good rare boots with ~35-40% triresists, not cheap. Needing a bunch of 4-5% all resist scs wasn't cheap early on in ladder. For a mang song's build, the budget form that hits max resists cheaper is to drop down to 75% FCR but keep full damage, while for obsession you can easily maintain that 125% FCR and have max resists, you just don't hit as much.
I really appreciate the feedback! Again, you're the father of this build, and I really appreciate the arguments you made in favor of maxing bind demon - you were 100% right on that.
My argument for obsession is that what it gives you removes the need to hunt for the truly expensive pieces, like tri res 35+ boots, all res scs with key mods, and 2/15 caster ammys before many players are 93 and stopping to craft ammys in bulk. It gives you so much, and enables flexibility elsewhere.
I do think that mang song, 3/20/2, and 2/15 will put out more damage, but it's just my take that it's not big enough to justify having to hunt for the resists/fhr/life.