To book or not to book european campsites

Reading other members remarks on european campsites   is it better to book sites or turn up on spec. We are looking to travel next winter and spend a few months in Spain and Portugal. I thought about booking sites but the only thing is for how long at each site. As this will be my first time touring aboard I dont know the sites. I have applied for an ACSI card. So best to pre book or turn up on spec?