You're not accounting for the fact that his arcane orb is sustainable via ApoC. Skorn will only allow you to maybe 1 more orb than your full AP will allow (with only Apoc on helm), unless you're only ever shooting into massive quantities of enemies.
Gotta account for casting sustainability. As long as your AP stays full with your APoC, OP, you'll be better off running a high black damage mainhand with some LL and maybe APOC (unless you are solvent where you are). That said, if you have a high enough crit rate to get by with JUST APOC on helm, then by all means, Skorn it up. Those things hit HARD. But I tested one and couldn't sustain casting with any big-hitter spells (meteor, orb). But my Crit rate isn't insane either, so thats' a different variable that I, personally, had to account for.
Not sure what your budget is like, but a wand with the following would be your best bet if you need to keep APoC:
Black damage (Min/max)
Int if possible, but that's a hard find.
If you don't need the extra ApOC that's even better, because then you can get a slower 1h with a higher base damage (hammers, axes) and slower attack speed.
However, if you're running an echoing fury right now and your attack speed isn't burning through your AP, I highly doubt a wand will do any worse...
No AP = no casting = no damage. Doesn't matter if your hitting for massive numbers if you can't keep the casting going.
What I said is that no one handed setup would do more damage than a good Skorn per cast... not that it would be better in the long run.
I'm sure you can make Arcane Orb casting sustaintable via APoC....
but that will consume at least 2 affixes on your gear.
With a slow weapon, you may use a good Helm with APoC and make it sustainable exactly the same way.
That said, If you have more damage in a single cast, then you are free to move around more and still do relevant damage.
I don't think it's a easy decision anyway.
1 handers with big APoC has more DPS if you have enough APoC and time to shoot
2 handers has better Damage per AP ratio, bigger damage per cast, and you're safer as you can kite more.
If you can be standing and shooting 1 handers are better assuming you can make it susteinable because raw DPS is higher.
If you need to move around a lot, 2 handers are the way to go :)
I can easily see this build working in both setups, most likely it's player preference.
this build works well with any "Archon gear"... just swap the Helm for something with APoC and you're in for some serious Arcane Orb Spam.