Do you tend to sleep warm or do you tend to sleep cold?
It can be downright cold in May! And as mentioned before, alburgues rarely turn on heat. Some will be frosty cold. Others will be so warm you'll want to open a window.
I would not go in May without a bag, although I WOULD go in June with only a liner. If I had to toss a coin, I'd say leave the bag at home and take a liner.
As far as make goes, I bought a Marmot Pounder Plus. It weighs 1.5 pounds and I've used it on two Caminos and I love it! It's lightweight and warm. I originally purchased the "Pounder" which only weighs 1 pound. But I tested it at home and froze all night...
If you are taking a poncho and layered clothing, you'd probably be just fine with a liner because if it WERE cold, you could layer on some clothes, and pull the poncho over you. You'd be surprised how much body heat those nylon ponchos hold in.
In the end, it's a personal choice and nobody can really predict what the weather will be. I only know that on those very cold, windy, wet days, nothing beats crawling into a warm sleeping bag after a 6 hour walk where you're chilled.