Hi Jane,
I haven't done the Interior Route, but from what I gather it's definitely a 'road less travelled': beautiful, rural, Basque... but also quite isolated (not many other pilgrims, or albergues), and you also have to watch out for weather conditions.
So I guess it depends on what sort of 'experience' you're after: if you enjoy walking alone, don't mind paying for private accommodation, and are comfortable with fickle winter weather, then do the Interior… On the del Norte, on the other hand, you'll meet up with more pilgrims and have the option of staying in albergues/ refugios, so less worry about planning things in advance, as well as having the chance to experience some great historical cities and beautiful landscapes/ seascapes along the way. You can also do the Camino del Salvador from Ovideo-Leon to catch up with the Frances a bit later if you so wish.