I've stayed both and indeed both are with more reasonable price than most others in Amsterdam, along with the Tribe which is just next door to Mercure Amsterdam north. Room wise both are quite new and of modern style.

If both are of the same price I'd probably go for Mercure North, the walk to the metro is slightly shorter than the one to Sloterdijk. If you are planning to use GVB day pass in Amsterdam, metro is included but not the train which is between central and Sloterdijk.