Hi MaryJoan. I took the Alsa bus from Santiago to Santander, it was cheap because I booked ahead and am an older person. It is a beautiful trip, at some points nearly heart-stopping because of the height of the road above the valleys below, but it is long, and when I travelled in 2015 there was no on-board toilet as you might have expected there to be provided on a long-distance coach.There was a stopover in Oviedo when all onward passengers had to leave the bus and wait for it to return later. Additionally, luggage is stored in the compartment accessed from outside and as there are frequent stops this made me a bit uneasy about the security of my backpack - but that's probably a neurosis hangover from having lived in a city for too many years.
I stayed overnight at Hospedaje Magallanes (listed on booking.com) and it was cheap, clean and comfortable. I know that other pilgrims who have stayed there share this view.
I hadn't considered taking the train to Santander in 2015 but, based on the posts above, it would seem that that would be a more comfortable option, although you would probably miss out on the spectacular coastal scenery of the bus route. Buen camino!