Shatter Armor or Frost Breath on a Widow
I'm assuming you mean Arctic Breath from the Leviathan PPP. I do not know what your experience has been, but I personally get sick of jumping in and out of the mix to maximize ranged cones and PBAoEs on melee characters. For that reason, I think Arctic Breath would be better suited for a Fort who has Dart Burst, Psy Scream, and Psy Tornado or something to that effect. Blasting AoE style. That's all personal taste, of course. A chip in AB's favor is the -def.
But I digress. I think Shatter Armor is a better choice. It's a great compliment to follow up, and as you say it would come in handy against AVs, and other large targets. More over, I think the power choices in Mace Mastery are more diverse, so you can cover other situations should you choose. The -recharge on Web Envelope is only -20%, but it'd still be worth firing off at said large targets. Disrupter Blast could make up some of the AoE you were considering, though it's not as good as the options in Leviatian.
@Gilia1
I play heroes on Champion.
I play villains on Virtue.
Would Shatter Armor or Frost Breath be worth taking for a Widow? (If not, I'll likely go for Darkest Night, so you might factor that in.)
Frost Breath is decent (-15%) short-lived (10 seconds) AoE -Res, and Shatter Armor is better (-20%) and sustainable (20 seconds) single-target -Res. Stealth Slash + Follow Up + Shatter + Placate + Slash seems like it would be a good way to show an AV you mean business (I'm sure the rest of the team would appreciate the -Res), and Follow Up + Frost Breath + Dart Burst + Spin sounds nice for team steamrolling and any large spawns when solo.
Shatter Armor seems like it might be worth it, but this is still "on paper" at this point, and I can definitely see how such a long animation would make it less appealing--it takes about as long to animate as the whole rest of your attack chain combined, if you include the two redraw times it forces. If it's borderline, the added damage from your team might make up for it, but on the other hand Darkest Night could significantly help survival against those same large targets, as well as Defense debuffers.