WMG' is more focused on business relating to science/engineering/manufacturing as far as I'm aware of, whereas WBS is a much more traditional business oriented one. WMG however sounds fantastic, considering the breadth and innovation of courses on offer. WBS, as you would already know, is world renowned as being one of the world's leading business schools.
Either way, you'll get a Masters from Warwick University, which in itself is a huge accomplishment. Hope this helps!