With a roster like that, it's hard to waive anyone. I think the choice comes down to Sierra and El Duque, as you said, but also D'Amico, who I'd value at about El Duque's level. That doesn't matter, though: Sierra is the one to drop, in spite of his currently hot bat. Two reasons: first, he won't play every day once Edgar returns; and second, you really don't need seven outfielders...
I know your problem, having seven outfielders, seven great outfielders. You don't want to drop any of them but you have to if you want to stay at the top of competition.
Don't be afraid to trade one of your top OF (and maybe another one) for a top closer or an upgrade at other positions. Your OF won't suffer much anyways and other teams might need help for their offense.
Juan Uribe is available as a FA in my league at the moment. Should I get him for Valentin or Cabrera or something? Valentin has hit a couple of homers recently so I don't want to get rid of him just yet really.
Uribe is the real deal. Give him another year or two, and he'll be the undisputed best shortstop in the NL, and he's pretty solid already. Cabrera and Valentin are also good, but I think the best combination of those three guys would be Uribe playing at home, and one of the other two playing when Uribe's on the road. If you like Valentin over Cabrera, drop Cabrera (not that he's shabby, either).