It's certainly a good week for Stewart and Pennington both.
Stewart faces Cinci, who causes the least trouble for opposing QBs. Cinci is dead last in sacks, tied for 3rd in most touchdowns allowed, and only has 8 interceptions on the year. About the only thing in their favor is that they only allow about 200 yards passing per game.
Pennington faces Buffalo, tied for worst in league for interceptions (5) and tied for 3rd in most touchdowns allowed. They are in the bottom third of the league in sacks. Like Cinci, they are a little stingy with passing yards, allowing an average of 216 per game.
Beuerlein is a good pickup, certainly, but he could use a week to shake off the cobwebs, especially facing Indy. The Colts are 2nd best in the league defending the pass (168.5 yds/game), and in the Top 10 in defending touchdown passes (11 allowed).
Beuerlein's matchup in Week 13 against San Diego is favorable, as the Chargers are tied for 3rd in touchdowns allowed (19) and are the 3rd worst in the league in passing yards allowed, at 266 yards per game.
If you get Beuerlein, I'd start Pennington this week and save Beuerlein for Week 13.
If you pick up Stewart, you may as well start him. This is the best matchup he's going to get.