Hello,
Katana isn't an AE set.
And buils wise, there's not a lot of place for flashing steel.
For me good ae set are : claw or spine coupled with dark or fire.
and the outsider : fire/sd/fire.
Now kat is a strong and fun set to play, if you like it it perhaps you should look toward a kat/dark combo 
Echoing a bit...
Katana isn't an AoE set. Claws/Dark is the better AoE machine.
Katana/Electric would probably have the edge for survivability if built for it, but you're trading damage output for that survivability. And you could probably be pretty solid with a well-built Claws/Dark.
Neither Cloak of Fear nor Oppressive Gloom are "almost the same benefit as softcapped melee" and lethal. They aren't particularly reliable - feared minions will still attack you at a reduced rate if you hit them for damage (which you're doing constantly with Death Shroud), stunned minions can wander out of the effect and attack you. They only mitigate minions, who aren't really big damage dealers anyway. On a Claws, you may want to remain mobile to take advantage of your cone and ranged attacks, plus giving yourself active mitigation through movement, rather than just planting yourself in the middle of the group for your damage aura and mitigation auras to take effect. Plus Oppressive Gloom causes you damage - still worth it, but it's worth noting.
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Both of these combos look rather interesting to me for mass AE slaughter with their damage aura and decent PBAE attacks. Wondering which would have the edge in terms of survivability?
I'm not massively wealthy, so wouldn't be purpling them out immediately. Looking at the base resist numbers, electric armour seems to have slightly better resists, and paired with katana, would be easily soft capped vs melee. Elec also has a toxic hole which could be an issue? On the other hand, dark armour has a superior heal, CoF or OG would provide almost the same benefit as a softcapped melee as they essentially mitigate the majority of melee attacks, but has much worse endurance issues to deal with, KB hole is not an issue with the appropriate enh.
So which of these 2 would be a better AE machine? both have the damage aura, my understanding is that Spin is better damage, shorter recharge over The Lotus Drops, and is enhanced by Followup even further. Flashing Steel has a very wide cone, but it's damage is rather inferior to Eviscerate. Not too interested in either of the knockback cones.
thanks!