Speaking as a relative neutral here, I'd argue that Barcelona has 2 world-class defenders in Pique and Puyol. Mascherano just doesn't impress me as truly world class (would I put him on a world XI as a cb? No.), so I'd say Barcelona needs another good centerback. The young players mentioned could well get decent playing time next year, depending on what happens with Puyol's injury/recoveray (keeping in mind that he isn't getting any younger), Pique's head, and the transfer market.
Of the 3 young players mentioned, I've been most impressed by Bartra. I'd give him the best chance right now.