Hi, Eilish,
Last year on the Primitivo, I had a Vodaphone sim card. The only time I remember a difference in coverage was in Berducedo where I couldn't get coverage and my movistar friends all had it. I think that though vodaphone and the other companies have made great improvements in their coverage, Movistar still wins. But I really didn't have any complaints with Vodaphone other than that one day.
As far as deals go, I think you will find that all of the companies have "deals," most of which are hard to compare with each other and frustrating for the consumer. But I got what I think was a good deal last summer on Vodaphone. Since at the beginning I was walking with another person who had Vodaphone, we decided to get the same package so that we could call each other for free. Then there were 2 GB of data, I think, and maybe 50 minutes (or maybe more). It lasted for a month and cost 15 euros. You could add on another month and not lose your minutes at any point in time. So since I was walking for seven weeks, after the third week of the deal, I added another four weeks, so I could carry over my data and minutes till the end of my walk.
The data was more than enough, since wifi is everywhere.
And though my memory is not my strong suit, I am pretty sure the card was called Super Yuser.
The only other thing you might want to consider is that in my experience, the Vodaphone people in stores from the Ebro Delta up through Santiago were friendly and helpful. In my other years of walking caminos, in which I always got Movistar, I have never come across a helpful and friendly Movistar employee, but that may just be my bad luck.