I guess it kind of depends.
If I'm actively hiking and need to carry supplies with me still, then I would go with a poncho simply because it will cover more and I can just toss it out when I'm done with it.
In general, though, I prefer raincoats or just an umbrella even. Covering up entirely always makes me feel clammy and sweaty...even when it's cold...so I like just being able to peel off the wet clothes later and switch to something dry when I get the chance.