Ponchos are a preference for many walking a Camino de Santiago. They cover your top half and, depending on the style you purchase, they can cover you and your backpack.
There's also an option to get a poncho that only covers your backpack. And, some packs have a poncho style covering built into them that you can unfurl in the event of rainy weather.
As with everything you'll pack, weight is a consideration. But, more important than weight for some is comfort. When I walked the Via de La Plata, we only had 2 days of rain (in 7 weeks). At the time I was walking, some friends I met on my Camino had a friend walking the France Way. It rained for 30 days straight for the friend.
I packed a poncho, and was glad to have it for my two rainy days. I wore my rain jacket and then wore the poncho over me and my pack. I stayed dry from the rain, but got rather sweaty with so many layers of clothing and jackets. So, it was a trade off.
Pullovers are also great. Especially the ones that pack tight into their own pockets.
As with so many items. I highly recommend that you get out and hike with your selections before heading to Spain. So, on a rainy day, put on your gear and wear a jacket, see how it goes. Put on your gear and wear a poncho on another rainy-day hike and see how that goes. Practice hiking with it in different combinations until you find what you like the best.