Grignard reagent reaction mechanism: Which bond breaks during the decomposition of the adduct?
As can be seen above, the end product is the same whether we split the $\ce{Mg-O}$ bond or the $\ce{C-O}$ bond.

The problem is, then, which pathway is more favourable?

This, I have no idea. Maybe both pathways are favoured. It doesn't matter because they give the same products.