I know both forms are correct, but I wonder which is more formal. Is there any difference in levels of formality?
I would personally go with more clear, if only because there is a common phrase that goes, "Things could not be more clear." But on the other hand, there is also the set phrase "clearer than day." It truly is a matter of personal taste, but I always felt clearer is slightly awkward-sounding -- and that more clear has a pleasing cadence. I'd say, for formal writing, you can't go wrong with more clear. I think the Queen of England would agree.