The Stellers are allowing 15 ppg. The Titans are allowing 13.1. I would pick the Titans over the Steelers at this point. And IMO neither one would beat out defenses like the 00 Ravens (10.3 ppg), the mid-80s Bears (11.7 or so ppg), or the 77 Falcons (9.2 ppg).
OK, I'll play. The first paragraph here will be my personal view, but beyond that, we'll go straight to numbers and nothing but.Here's the thing. The Steelers' D ppg allowed has also been skewed this year a bit by bad calls, bad bounces, and the offense turning the ball over non-stop, giving other teams a short field. Now, the first two you can discard as being something that befalls every defense, but watching every snap as I do, I can tell you that it has made more of an impact on this year's defense as any Steelers D I can remember. The bottom line is that the offense and special teams are doing them no favors. Not only is the offense turnign the ball over too much, the punting game has been putrid since Sepulveda got hurt. berger was OK, but Ernster is awful, and I swear to you, at least 3 or 4 punters playing against Pitt this year have averaged about 55 yards a punt.

It seriously seems everyone has a career day against them. I don't know where to find this stat, but I know you're a stat guy, if you can get the avg gross yardage of Pittsburgh's punt vs. that of their opponents, I'd actually probably bet even money that they're dead last in the NFl in differential. If not, they're close.
The final stat does have a big effect, though. The Steelers have allowed a defensive TD and 2 safeties this year. Those aren't on the defense, and the Titans offense has not allowed a point the other way all year. Right there, that's 11 points the Steelers D is not responsible for, which drops their true ppg against to 13.9. Also, the Steelers have 4 more turnovers than Tennessee this year. That accounts for a good chunk (if not all) of that .8 ppg difference. Pittsburgh has actually allowed fewer TDs (13 to 15) and scoring opportunities (30 to 31 TDs and attempted FGs) than Tennessee. A big difference in the ppg allowed is that opposing kickers have hit 16/17 FGA against PIT and only 9/16 against Tennessee. That right there is a difference of 1.8 ppg.
Now, the one stat defensively which is the most bullet-proof in terms of showing the true stinginess of a defense is yards allowed per play. That takes everything else out of the equation as well as you possibly can.. starting field position, points allowed by the offense/special teams, # of snaps taken by opponents, etc.. etc.. It's not perfect, but it's as good as you can get.
The Steelers are currently allowing 3.83 yards per snap, which is the lowest # in the NFL since 1979. Better than the 2000 Ravens, better than the '85 Bears, better than this year's Titans.
So, for that alone, I'd say they're in the conversation of best defenses in the past 30 years. I'd still probably rank them a notch below the '85 Bears or 2000 Ravens, but they're close. The rest of the season will tell the tale. I'd take them over the Titans this year, though.. and that's with 2 of their top 3 corners out the past few weeks.