If you're lucky enough to have a brass stemmed valve, and it is nicely chamfered, I'd go with the nut and olive. If on the other hand you're dealing with plastic, it has to be a tap connector.
It always pays to have a close look at the valve, or any tap for that matter. Any chamfering reduces the surface area that the washer of a tap connector can seal to. There's many times I've had to carefully use a file. So if it's possible to use the nut and olive method, I prefer it.
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