There is no doubt in my mind, that for a first Camino, I would always recommend the
Camino Frances, it is very well-supported and has a fantastic infrastructure. The first day from SJPP is easily one of the hardest days on any Camino route. After that, there is only one significant climb that lasts a few hours.
As others have mentioned the first week of the Norte is very up-and-down, with less infrastructure in terms of stopping for a coffee or as large a selection of Albergues as can be found on the French route. That said about 40% of the people I met when I walked the Norte, were doing their first Camino, and to the best of my knowledge not one of them gave up on the route as they had found it too hard.
I think two things come into play as much as possible plan your route in terms of kilometres walk to per day, the other thing is mindset, if you are determined to adjust your pace you will get there. Do not be discouraged when people tell you it is challenging,