Teamfight - Dust or Ward?

Answer depends on the hero and the situation. Most heroes with Shadow Blade will be initiating with Shadow Blade, in those cases, sentries are probably best but have someone carry one dust in case they try to escape with it. For Slark and Weaver, don't even bother with Dust. Slark purges it, and when Weaver ults he purges it. Riki can purge it as well when he gets Diffusal.

Honestly, sentries are just usually better, but Dust is usually fine early game. When it comes to late game sentries are better (generally speaking).

It depends on whether you need to avoid getting ganked or avoid someone escaping.

So if you're dealing with a Riki, then wards

Dealing with a Glimmer Cape, buy dust.