From a cycling point of view, I can't comment as I have not cycled in either. But I have walked in the mountains of both, including walking on the roads.
The Spanish may be crazy on the road, the Italians are worse, although the Italians tend to be slightly more considerate towards cyclists, but nothing like as considerate as the French.
In terms of quality of the roads the Italians are better than the Spanish, but the Italians have a lot more traffic.
Personally I prefered N.Spain to N.Italy, as it was more remote, more spectacular, fewer people, towns, infastructure etc.