I would choose the £600 spending money.
My reason is that I would want the whole family to enjoy DC and although you DS won't cost anything it does mean you will have to split up.
Also £600 is around $900 and that could go along way on stuff you can do together.
We need 3 swims and 1 non swim, is that still that price? I think we;re booked for the 18th currently...