I looked at both before my camino in 2012, they both have something a little different from each other.Eroski had just started updating its diagrams but was only up to the first 2 etapas however like you said Gronze seemed to give a little more advice on the variants.
I ended up using advice and distances from both, and dont forget the replies from pilgrims that come with each etapa on Eroski they can offer little insights or secrets that are not common knowledge I found out about one albergue at Boo before it became listed
for that stage. I know of another one still not listed but mentioned by the pilgrims comments on that etapa. So i would pick which diagrams, if you want suits you best and take comments and notes from each. In the end I didnt take anything I had been on this topic so much I left it too my memory to hopefully remind me of distances and places, which was fine in theory, but after San Sebastian I realised I didnt have clue, so i had to make regular visits to internet cafes.
And just realised that i had only answered part of your question, the alternatives as I remember are indicated on the ground. Gronze tends to list most of the variants and compares them against the main routes also giving distances. I know the cicerone guide lists the variants and gives details on them as well.
