Depends on the brand(s) of primer involved, really. I at least can't give a better answer unless you specify those
As long as it's actual primer I don't think you'll actually notice much difference.
NB: Of the GW spray cans, only Chaos Black and Corax White qualify as primers - and Corax has a for-best-results manual a couple pages long that isn't included with the spray can.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.