Well if it has to be one or the other then I say go for the magnum, 12 syns I think and crunchie's are 11 but magnums are bigger so worth the extra syn. Only if you really really really want it though.
I know its been and gone and digested now, but I would have picked the Crunchie Blast on the basis that white chocolate is a waste of good chocolate. Ordinary Magnum might have beaten the Crunchie though.