Letang, Buff, Chara, Green, Yandle are the D available.
Tavares, Spezza, Toews are the Cs available.
Any C lower than that are not in the conversation unless they are just the plus in a bigger trade.
The team wanting D does not value Letang fully because of injuries, so he's out. So is Green.
Buff/Chara/Yandle vs Tavares/Spezza/Toews
Narrows down to 3 choices a side, and the rest is personal opinion.
My personal opinion is that I wouldnt want Buff or Spezza....so why not just swap those 2