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For a rest day, wherever you are when you find that you need a rest. For a sightseeing day, depends on what you are looking to do and what your interests are.
Pontevedra is the bigger city, with more sights to see, museums, art galleries...
Another vote for Pontevedra but since you mentioned Caldas de Reis as a possibility, I wonder if you know about and/or have considered the Variante Espiritual? It’s a popular variant to the main route, starting just after Pontevedra and finishing...
If you do take a rest day in Pontevedra, you can try to schedule your walking to where the walk into Santiago is only 7.5 or 10km on that day. That way, you can arrive in Santiago without being too tired.
Pontevedra makes sense if you want do some things like shop, but Caldas would be a nice place to actually rest. Then you go on to Padron and the next day into Santiago
If you hit Padron on the day of their big (huge) outdoor market, that could be interesting. I went through on a Sunday, but I don't know if it is every week.
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