Hi there, we've been to both islands. We usually go early June.
For both island I'd say May could get you mixed weather, as the resorts only start to open their doors end of April beginning of May for the season. Even when we go the resorts can be fairly quiet for the first week.
For us we found kos to be such a friendly place. But we were bitten to buggery! By mozzies. Even with the repellant stuff. We also didn't like the airport that much. Kos was hotter than Zante had been at the same time of yr. but when we got home and checked that yr. Zante had a heat wave at the same time.
Zante on the other hand is now somewhere we return time and time again. Rarely get bitten, but still use repellant. From beginning of June, in Zante we have found temperatures to be around 27c.... But getter warmer each day...
Going in July this yr! So hopefully hot hot hot!!