Both of them posses fantastic attacking skills but they both suck shit at defence. Zhirkov plays as left winger at his club CSKA Moskva, so he shouldn't be considered as defender at all. In Euro 2008 Germany conceded almost all goals from the left flank where Lahm operates, imo he's a poor defender too. It would make sense if Barca had a defensive right back, but having two attacking full backs could let the team be easily exposed in the back.