Mages are more consistent money in my experience (even with the cost of potions), since you're likely to get at least two an hour if you do it right. Also they're really good ranging (and melee) xp. However, abyssal demons are much more relaxing (you do not need to pay constant attention; there is no risk of prayer running out), and demons drop charms. I'd say just go for 85 (it's not too much more experience), and then you can kill whichever you prefer on a given day.