Hi,
Both beaches will be very crowded in August,you will find a bed on the Palma Nova beach you can't book one ,Illetas is much smaller and has a limited number of beds.
Personally I would still go to Illetas as it is far prettier , there 3 beaches there with a mix of sand and rocky shoreline.
If you are there around 10am you will definitely find a bed .if you book the restaurant for lunch it is excellent.
It will be Hot !!
Roger b