hello Greg.
I walked both the Norte and the CP (from Porto) last year. How to compare ? Just some thoughts that may help you decide - although I do not know what is important for you.
You have to walk very fast to complete the Norte in 22 days: it is app. 820 km. I do not know how fast you walk, but you have to be really fit to make it in this time period (it took me almost twice as long). The first part between Irun and Bilbao is rather hilly with steep ascents and descents, but very beautiful, so you do not want to skip this. After that, the walking gets easier. Big bonus of the Norte: you walk along the coast line a lot, and you can go for a swim if you want.
The CP (from Porto) onwards is definitely an easier walk than the Norte. From Porto onwards it is also more crowded than the Norte. I believe the CP is getting more and more popular because it is relatively easy going & I actually met a lot of people there who walked it as their first camino. The stretch between Lisboa and Porto is far less crowded (have not walked this yet, but plan to do so). Walking the CP from Lisboa is more doable in 22 days. Big bonus of the CP = visiting Portugal (I really loved that country).
Well, how to decide ? Eventually you walk both. Indeed.
Buen camino, Marc.