Coolant Pressure Cap.....17 or 20 PSI?

I was looking to replace the coolant pressure cap. BUT the forum/manual says it should be 20 PSI, BUT the cap on my car is 17 PSI. Always has been. But all the ones to buy on EBay , are all 17 PSI for all Aveos, and like 10 other cars.

So...does anyone know if it is 17 PSI or 20 PSI, or does it even matter??

I highly doubt it'll matter.