I crafted the shittiest Void, 10%. My DMG goes from 2732 to 2904 oh hammers, without conc. with conc, it seems to be the same, if not actually a bit lower of a few hundreds (but cannot see because both say 12K). So with that in mind, I'm thinking a 15% Void beats HOTO for sure DMG wise.
I'm not sure I understood what you said here...
You gain 172 hammer dmg when you swap hoto for void?
But then with conc, they both show as 12k?
If that's correct, then it would mean the +% magic dmg is only additive with conc
so hoto would give +3 hammer +3 conc, and a 15% void would give +2 hammer and the equivalent of +4 conc
But maybe that's not what you wanted to say...