I walked parts of both of those routes in 2021 and they are equally lovely in their own ways. However, I am going to have to be a bit of a party-pooper here. My apologies in advance…
March is a bit early for walking the Norte and probably impossibly early for the Arles/Aragon route due to seasonably closed albergues and snow. You certainly want to verify the availability of accommodations at that time of year for each of those routes, especially since Covid closed the Basque region’s public albergues in 2020 and 2021. The Somport Pass in Aragon will definitely have snow on it in March - you can bus around the hardest part, but do not be surprised if you have to bus all the way down to Jaca to find trails not adversely impacted by the weather. I’m not saying NOT to walk either of those in early Spring, just that you need to be very flexible and careful regarding weather and finding places to stay.
All that aside, if you walk the Norte, definitely start at the Bayonne airport itself, walking to either Biarritz or Bayonne on the first day after deplaning. It’s easy to do, especially using maps.me or a similar trail map application as the route is not clearly marked. From Bayonne, head to Biarritz via the city streets and then along the shoreline route to St Jean de Luz - the route that skirts the eastern side of the airport isn’t that picturesque.
If you can delay your walk to late March or early April, you’ll find the going a bit easier. May? Perfect!