as the one who is usually doing the filling, honestly i couldnt really decide. on the one hand, filling them with loads and loads of cum until shes dripping your excess is nice *bonus points for full females who could/will get preggers.* but on the other hand, covering them in it, as others have said, marks them as yours, or just further shows how much of a cum receptacle they are, as it doesnt matter if its in or on, they want all the cum, and that on its own is is hot. also, in terms of outside, if someone really really wants it inside, and youre a dom and you wish to punish them as a sub, cumming outside is an extra little element too
edit: final verdict for me though, i always like filling rather than covering