Hi Andrea, welcome to the forum!
There will be a very long list of pros and cons for each, and no doubt everyone who replies to this thread will have their own opinions. To simplify, I'd sum up the Norte as more of a "nature" experience, and the Frances as more of a "culture" experience.
Norte: beaches, rural landscapes, mountains... you will have to pass through a couple of touristy coastal towns and port areas along the way, but I personally loved being so close to the sea for a month. And you won't be lonely! I find on the Norte you are more likely to stick with the same group, as you don't have new 'waves' of pilgrims arriving at hotspots and there are fewer albergues in each town so you tend to see the same people every evening.
Frances: medieval architecture, churches, whole towns built around the camino trade, lots of fellow pilgrims and camaraderie, a sense that you're part of a very old pilgrimage tradition... however there are lots of tourigrinos, and people in general - a very sociable camino, which is, I think, the main reason some love and others hate it.
As I said, that's a real simplification! I'm sure whatever you choose you'll have an amazing time!