Marquez and Puyol both out for the second leg. Providing Chelsea keep 11 men on the field, they'll win the second leg. Barcelona have been eliminated on English soil three times in the last four seasons. Can't see this one being much different. Defensively, particularly away from home they're not great, and with key players missing, Chelsea should have more than enough.
IMO, Chelsea can probably afford to concede and still win. Doesn't really bother me either way, I'm not a Chelsea fan, but they're like robots at home in Europe, they just grind out results and I expect they'll do the same in the second leg. To be honest, it really wouldn't surprise me if Barce don't score.
From memory, last season, when Barcelona went to OT in the SFs, didn't they also have Eto'o, Henry and Messi in the squad? Yet United stopped them scoring with a defence that included Wes Brown and Owen Hargreaves. Barce are great with the ball, and generally tend to have plenty of possession, but over the last three or four years they've struggled to really make this count against English opposition. The only success they've had is when the English team's been down to 10 men (Chelsea, Arsenal).