It totally depends. If you feel that your opponents mostly use Physically defensive Pokemon, use Physical sweepers.
Or, if they tend to use physically based ones, use a Special Pokemon.
Mostly though, when it comes to sweepers, Mixed Pokemon are preferred, such as Infernape, Salamence, Lucario and so forth.
Or else, you could use a Tank, like Blastoise or Hippowdown, or even a Wall like Lugia (if you can get that), or Amoongus, and so on.
tl;dr Go Mixed. Mixed sweepers generally have better coverage and get the job done. Only downside is that most of them are frail.