Depends where you are coming from.
Santander is more forgiving and less hilly. From Bilbao is a lot harder walking.
If you want an interesting alternative of sorts, walk Santander to San Vicente de la Barquera and walk the Lebaniego to Potes and claim your certificate from the Santa Toribio Monastery. That would be a 6 day walk if you use the Gronze stages and include three days of walking in the mountains. The last day could be spent walking the hill trails above the Monastery if you wanted.
But overall I would say start from Santander, it's definitely nicer. If you want, you can shorten the first day by talking the train to the outskirts of Santander (saves an interesting walk through industrial areas) to the station at Barreda. After that you are out in the countryside away from the motorway and fumes.