Well I don't have 85 slayer yet but I'd say that I'd go with demons just because of the fact that with D Boots it seems that my drop rate is about 1/160, and with the whole drinking prayer potion thing I'd rather kill demons because you don't have to pay as much attention. Also with the price of the D Boots dropping to 289k in the GE I wouldn't really say their worth it anymore. But yeah I thought someone with 89 slayer would think D Boots weren't as common at 1/15...