I think there are 5 primary schools in Ontinyent. Schools are different in Spain to the UK as there is no real entity in the sense that the Headmaster has no budget, hiring or firing authority and is just voted in each year from the general pool of staff. Teachers have a job for life and are not rewarded for excellence or penalized for lack of it. Inspections are generally useless except for most extreme cases.
It sounds awful, but surprisingly it's not - well at least not in my personal experience. There are a lot of very dedicated teachers out there and some that......
I'd guess that catchment area may well affect the quality. For the relatively rural setting you are looking at I'd be keen that my child's peers are made up of his/her immediate neighbourhood.
Generally my feeling is that the disadvantages of education here are outweighed by the supportive environment (I'm talking about villages and small towns).
Your daughter is also going to be tri-lingual - how great is that?
Personally I'm against private education generally and in Spain even more so as it tries very hard to be elitist and normally comes with a strong religious bias.