It depends on your personal preferences and the purpose of your walk. If you're walking the Camino de Santiago, starting in Cadiz would mean that you'd follow a different route than the traditional Camino de Santiago, but it would still offer a unique and scenic experience. Cadiz is a historic port city located on the southern coast of Spain and has its own rich cultural heritage and natural beauty to offer. On the other hand, Seville is a vibrant and culturally rich city located in the Andalusia region and is a popular starting point for the Camino de Santiago.
If your main goal is to complete the Camino de Santiago, starting in Seville would be the most convenient option as it is a well-established starting point and there are many resources available to support you along the way. However, if you're looking for a new and different experience, starting in Cadiz could be a good option. Ultimately, the choice of where to start depends on your personal preferences, interests, and the type of experience you're looking for.