I'd play the Steelers D over the Broncos...my main reasoning being:
1) Game is in PIT, so Denver is traveling cross-time zones
2) Denver's offense not as big a threat as Pitt's, especially on the road
3) Pitt's D has been fairly good from a fantasy perspective this season.
4) Defenses generally play better at home than on the road
5) Broncos' D hasn't been the superior unit it was to start the season outside of a trampling of the abysmal Chargers