Having worked very closely with Mike Sohm at Magnus during the company's formative years, I can tell you that metalurgist reports show the Zwickey a couple of Rockwell points softer than the Magnus. That was then and of course things can change.
One should also take into account the metal characteristics if the two companies use different steel. No two steels react to "work" in exactly the same way.
That could infuence the way the broadhead reacts to a file.
Example... Zwickey going to copper brazing to strengthen an already strong head and the resultant heat treating that had always been done "in house".
The Zwickey's would be even stronger if they backed off on the spot welds and let the brazing do it's job.... spot welding actually removes/thins the metal along the edges of the laminated pieces, therefore creating potential weak spots.
I prefer Magnus heads as most know, but I also have a long history with Zwickeys and would fall back on them in a nano second if I had to... I like the Magnus strength and slightly larger size.
Of the two broadheads the standard Zwickey Eskimo is going to have a slight edge in the strength department... it's convex grind makes the difference, putting slightly more metal at the tip.
With the Zwickeys (I've broken a bunch... failure of the ferrule under compression) I found two things that really strengthen the four blade heads (or the two blade for that matter).
Most failures occurred when using short adapters... fill up the ferrule with a long adapter.
The other factor is glue used and it's bond.
Slow cure epoxy will make the Zwickey almost bomb proof but the gluing surfaces need to be pristinely clean.
Burning out residue isn't the way to do it. Think about it! You don't burn anything "completely" away. There will always be some form of redidue left over.
Make sure that any foreign material is removed by abrasive or solvent. There can be paint or oils from the machining process left in the ferrule.
I prefer to clean well with acetone, score with sandpaper and clean with acetone again. I use the same procedure for the adapters.
All the broadheads mentioned are fine products and a fella shouldn't feel like he's gambling regarless of his choice.
As a side note; As loyal as Single bevel fans are that style is unlikely to ever hold more than a minimal share of the market. Magnus and Zwicky will likely dominate as always.
I've seen the numbers.