PC Mongul is a beast, but Classic Strange has handled as bad before, and Mongul doesn't have the diversity that, say, Doctor Fate would, such as in the (Classic) Strange vs. Fate thread.
I'd go with Strange in general, with Mongul getting wins via CIS/PIS on Strange's part, or situational wins giving Mongul the drop (because, of course, if he got his hands on Strange for even a fraction of a second, it's over...).