Hi Mick!
I prefer to go to Arillas in May as the resort is 'waking up' and it is generally very quiet. The sea is not too bad, I usually manage a dip! Great time for walking as the day temperature is normally in 70'sF. You might have an occasional shower of rain.
I am going on 8th. May next year for two weeks, and then hopefully in September for three weeks. Sea is much warmer then and the resort is 'winding down' a bit.
Martin.