I'm not sure about Pronger-Timonen, they'll both be 36 years old. I wouldn't bet on both of them maintaining their level of play, especially after Pronger's injuries. As much as I dislike them, I'd rather go with Chara-Seidenberg.
I agree with you about the Caps' D, that's a solid unit.
Age and top defensemen are tricky. They've been solid but certainly could fall off a cliff at any time by this point. Pronger especially considering the game he plays and how much his skating has declined since his 20's. But the could also be pretty good into their late 30's.
Chara's legitmately a monster but I don't think Seidenberg is that great. He's a fairly average top-4 defenseman.