I have used both on the CF last year. I started with a poncho on the 3 rainy days I had before arriving at Sarria. I have to say I wasn't all that impressed with it, I seemed to be wetter on the inside than the outside.
My wife joined me in Sarria and brought out a Berghaus Packlite jacket for me, we had some rain during the Sarria SDC section and I found the jacket more useable than the poncho.
I think it's a matter of choice what works for me might not work for you, you have to give it a go.
I must add that I came across a pilgrim several times during the early rainy days I had, and she was wearing one of those yellow poncho's that you get given at Niagara Falls, I'm not sure but I think she said it had lasted her a couple of camino's