done HV Kos
Aegean Blue Rhodes (which is the hotel directly next to HV Rhodes, and they share the same beach and staff)
(and also Blue Lagoon Village Kos, 3 times)
All are lovely. Id go back to them all, but given the choice I'd choose Blue lagoon Village (hence we have been 3 times, and not back to HV kos, or Rhodes)
However in May (we have been to HV Kos at pretty much those exact dates) the pools will be really cold.
If you go in July / Aug the pools will be much warmer.
All will give you excellent holidays, but in May, all the pools will be just a bit too cold.
The Beach in Rhodes (which IS HV Rhodes beach is my least favourite of the beaches)
IMO Slides etc are very slightly better in Rhodes (but beach is poorer) and food is better in Kos, but only a little, and its definitely not as good as it used to be......
But you would still enjoy your holiday which ever you choose