Depends what you want, obviously. A classic stout with just bittering hops, or a stout with some hop flavour. You could use either hop for bittering, Magnum is cleaner and less noticeable. If you want hop flavour, chuck some Northdown in with 15 mins left.
Yeast wise, again it's down to taste. My favourite stout to date used US05. I liked the lack of esters or yeast flavour. But it might not suit you. S04 is popular. Liquid yeasts offer more choice.