Are there any rules that say you cannot do both?
Islanders are going to have to be buyers on defense just to survive the season unless Gervais and Witt are closer than what's being reported. It's asking a lot to expect Bryan Berard, Radek Martinek and some of these other defenders to be durable the rest of the way and history is not on their side.
Trading a low pick for a defender along the lines of a Brad Lukowich who can take Sutton's or Campoli's spot would not be a bad idea for Garth Snow. If the Isles do get into the playoff they are going to need some depth.
If this team is winning as they are right now and getting enough scoring to be competitive plus keeping shots against down, there is no rule here that says Mike Sillinger cannot be traded in a sellers move for a young player and a pick that can help along the way.
Hunter, Fedotenko Comrie and Satan are possible trades where it get's interesting because those are players who are contributing to the winning streak. You trade them now you are playing more for the future unless it's a lateral move to fill a spot.