Both represent risk as well as reward. Carrington, the former Bills defensive end, he's a long and powerful interior pass rusher who can play 5-technique, but is coming off a season-ending quadriceps injury that apparently was enough for the Bills to not want to make him enough of an offer to stay. He played well in 2012, and a solid 2013 could have had him in much more demand in this year's free agency class.
Thomas has the reputation of being something of an underachiever from his time in San Diego. He was benched at times with the Chargers, but did show he can perform very well both as a starter and a contributor.
Judging from afar, he has all the makings of a change-of-scenery kind of guy; get him into a new location and he thrives. At the same time, he has all the makings of a player who just doesn't have the consistency to make himself an outstanding player game in and game out.
Whichever direction the Steelers end up going with these players, they need depth along their defensive line. It's debatable how much overall value either Ziggy Woods (signed with Jacksonville) or Al Woods (signed with Tennessee) bring, but clearly the team was ok, to some extent, letting them walk. One can logically assume there's been a replacement plan in place from the beginning. Maybe that's Carrington, or Thomas, or even both.