You want to apply AmpDmg, so Battle Cry is a no-go. Infinity gives you the benefits of Battle Cry without taking up the Curse slot, and you save up on needing to stack a lot of Attack Rating to hit targets.
Infinity also overrides enemy Conviction so you can't be affected by it, as only one of the two Auras applies afaik.
The setups are pretty vastly different, if I'm honest. You're underestimating how much MF you can get from just stacking 7MF charms + Gheed's. You don't need to do more than rub your testicles and you can hit 300MF, between Enigma and War Travs and charms you shoot past it.
Add in Goldwrap and other gear pieces and a swap and a Renewed charm and you can easily hit 450~MF on your DPS set, with over 600 on swap.
I think you should fuck around with the D2Planner more and determine what you're trying to maximize. Pick a variable, and maximize it. Like just make a MAXIMUM MF set (and I mean: MAXIMUM MF). Then, start removing bits and pieces to make room for DPS.
Soon enough you'll find the right balance.
There is no universe where that involves using an A5 Merc with Dreadfang, I can guarantee you that. Surrendering Might Aura + Polearm/Spear Aura for a sketchy Amp Dmg and Battle Cry override is just not it.
I actually just tried the Dreadfang on swap idea, and honestly it felt way too clunky. Tele into a pack, swap, Frenzy to try to proc Amp, kill a few, swap back, start Whirlwinding, move two feet, scan for a unique, repeat. Over and over. It absolutely slowed my kill speed.
I don’t think you’re putting enough weight into how awkward that loop feels in practice. The simplicity of having Amp auto cast repeatedly from the merc while I just tele stomp and start Whirlwinding is priceless. That passive uptime, with zero input from me, keeps the flow smooth.
With the swap method, you’re standing there Frenzying with weak DPS, hoping you don’t get chunked, waiting for Amp to proc, then swapping back. It feels slow and unsmooth. The auto application from the merc just plays better.