hamsterley is good for downhill and 4x but the red and black xc routes are poor (the red isn't really worth doing and i would grade it as a blue (nothing technical at all) and the black is quite short ~10km if a remember correctly).
Kielder forest is much better for the xc routes both the red loop and the black are very good. Easy to spend a day at kielder doing parts of the trails and a bit of sessioning.
So my vote is for kielder (there is a chance i might be there on tuesday as well 😀 if the bike shop get the cartridge for my forks in tomorrow)
James