It would be helpful to know if your objective is, say, "beach relaxation" or "hiking", or "living like a local in a forgotten little village", not that there aren't other things to do at those destinations. I'd also like to know where you live, because many non-coastal Americans have an inflated idea of the beauty of the beach and bathing options in Italy. I think both locations are hard to "go home" from, since you cannot sleep in them and fly out after waking up there in the morning.
I personally don't swim in the Mediterranean, as some of my doctors won't! But the water is pretty cold in May, anyway. That is an opinion, not a fact or a rule.
Edit: They are not comparable at all, but as a travel practicality, you might want to search here (top left or top center Search box)for comments on visiting Mont St. Michel (MSM) or the Atlantic Coast of France, including St. Malo and more glamorous places. I haven't been to the Channel Islands, but that might be of interest by plane. Grey weather possibilities, though.