When it comes to these sorts of contract decisions I often think back to Nomar Garciaparra's decision to turn down the Red Sox's $60 million contract offer. He was hoping to get a contract like Derek Jeter (10 years $189 million). He ended up getting hurt, then traded. Never made close to that amount afterwards.
If I were Marcus I would take the money. Sure, he might make more money if he waits. Then again, if he gets hurt that's a whole lot of cash down the drain. Keep in mind he's been hurt each year he's been here. Fortunately, he keeps bouncing back.
Another example might be Isaiah's injury situation...although, that's not quite the same as we don't know if he'll make a complete recovery.