I think it's very difficult to comment on them as mentors at all, because you really don't know how much the judges actually contribute and how much is all producers. The teens have been 'mentored' very well and given great song choices, whereas the young adults really hadn't, but whether that's down to Britney and Demi, it's hard to say.
In terms of judging Britney is awful. Fantastic facial expressions
but an awful judge who just seems to comment one line of whatever random thought comes into her mind and that's it. She's like a robot. Demi's a very good judge though. She's able to express herself quite well and she doesn't hold back for anyone which is what she needs. Her interaction with Simon can border on cringeworthy at times, it does add something to the panel though. (Also I do have some bias towards her because imo she's one of the most talented judges they've had on X Factor... Love her voice <3)
Bottom line for me would be keep Demi, and replace Britney for the next season (Though they'll probably end up doing it the other way around...)