We've had Skrtel, Agger and Carra in our defence for the last few years. We've had a lot of problems in that time in certain situations, because they had their limitations - not only individually, but as CB pairings. This season, we invested heavily in that area of the pitch, and I think it's reaping benefits, and will do. Not since Sami left have we actually had a presence in the middle of the park. I mean you watch Sakho against Altidore last week, and there was no contest.
I honestly think Sakho and Toure need to start all games for us if there're no injuries. It's about time we locked down our defence, and those 2 have shown massive improvements (and the qualities we've lacked) this season. Of course Sakho's still young and learning the game, in a new team and league. But he'll get there quicker if he's got the natural leader in Toure next to him with all his experience and quality himself. Not to mention that it also means we'd have 3 French speakers in the middle there - so communication would not be a problem until Sakho learns the language.
If it's 3 at the back, then I think you go with Agger, because he's a top defender as well as comfortable on the ball himself (comparing him to Skrtel). But I don't think you can knock Martin this season, so that's a tough one for BR.
I guess the big tests are coming up in the next month or so. West Brom are a decent team these days, but we have Arsenal away in less than a month's time, then Everton. That's players like Ozil, Giroud, Ramsey, Lukaku, etc going at these guys. Clean sheet in the Arsenal game, at least, is hard to see, but you have to wonder who handles their fast football better (especially because they'll crack through our midfield, leaving our defence exposed fairly easily, unless we play to shut them out first and foremost) - I'd be more comfortable seeing Toure and Sakho line up. And against Lukaku, it's a no brainer that they start if fit as well. We'll see what they're made of in the next month or so though.