I have used a Fairydown pack which had a front panel as well as top loading. My current camino favourite, the Deuter Guide 45+ has a side opening for access as well as the top opening, but it is not a full panel arrangement. I also have an Osprey Aether that has a front panel. It is smaller than the Fairydown it will replace, but still a very large pack by camino standards.
My take on this is that panels are great for larger packs where they provide advantages in getting access to the depth of the pack without having to unpack it. I don't think that they necessarily offer big advantages in the sizes I would be thinking of carrying in Spain, except perhaps in the depths of winter.
There might be other reasons to choose a panel pack, but accessibility seems to me to be the primary issue.