On Sarges vs Voxes, though, it is often cheaper to use Veteran Sarges. They cost only 1 point more than a Vox, but a Master Vox more than makes up for this 1point if you take it. Indeed, a Command Master Vox, AF vox, 2 Squads vox and a Command Section vox means you end up spending 40 points, IIRC, on vox links for the bare minimum of an army (HQ and 2 Troops), while a 3 Veteran Sarges would be 18 points. Although they'd only be LD8, rather than the 9/10 you may get with a HQ.
Still, like I said, I'd never take them on anything but infiltrators or AF Squads. I expect these to be outside command bubbles, (or in the case of my Light Infantry, Command Sections often get shot). In static line infantry, I'd never take them. Ever.