I'd keep MdB. I heard a story once that Hinkley and Port keep Kane Mitchell around on the basis that it's unlikely he'll play many games, but that his effort and influence on the training track is a good for their younger guys. If we are rebuilding, then I reckon guys like MdB are good to have around - especially if there is some truth to the rumours that there has been a bit of a schism in the playing group. Also, if we do lose Mayne and Ballas, then there could be a role for MdB in the forward line. As Polly said in another post, there are shades of Picken there if we could just use him in the right way.
As for Darling, he's a mid-key forward who kicks goals and is entering his prime with something to prove. Sure, he's squibbed a contest now and again, but there's a chance he'll learn from that, and it doesn't seem to stop people from wanting B Hill. He squibbed one a beauty against West Coast this year.
Of course, the Darling trade would never happen (Ballas has very little trade value), and I'm grateful that I won't have to spend the next 6 years defending Darling on here.