Capture towards the center vs. undoubling There is a spoken advice to capture with the pawns always towards the center. This situation let me think for a while. Is undoubling here not better?
In this particular position both ...ab6 and ...cb6 are reasonable.

Generally speaking I would prefer to take toward the center because the doubled pawns are not weak at all when they have neighbours and the enhanced control of the center might prove useful.

Here however ab6 has the minus that it allows White counterplay thanks to the outside passed pawn after a4-a5.

The evaluation of this position will depend on Black breaking through or not on the kingside, with ...Rf3 or ...g4. My intuition is that White should be able to hold. If that's correct, the best choice might very well be to limit counterplay with 1...cb6. But once again, this is merely due to the specifics of this particular position, and not because undoubling would be better than taking toward the center in general.