Santiago de Compostela

Both areas are excellent destinations. How much total time do you have for this trip? What are your precise travel dates? September is a transitional month, weather-wise, but Seville could be extremely hot even at the end of the month. This year it hit 90 F or higher each of the last five days of September. You can check actual, day-by-day weather stats going back at least ten years on the website timeanddate.com.

There's precious little risk of really excessive heat in Santiago de Compostela, which is known for lack of sunshine even in mid-summer. Last September it managed to get up to 79 F twice, but some days stayed in the 60s.

Neither of these destinations will be particularly easy to reach from Portugal. Ground transportation links between the two countries are limited and infrequent. It is doable, but it may cost more time than you're expecting, and Portugal has a lot of worthwhile places besides Lisbon and Porto. Actually, if your trip is short, I wouldn't go to the Algarve--but I'm totally uninterested in beaches.

If you’ve never been to either, I’d choose Sevilla because it has more sights. I love Santiago de Compostela too, but other than the cathedral, the historic center is fairly compact. Two nights would be sufficient. However, if you had a car, then there are some decent day trips one can take from Santiago. I was in Santiago and that part of Spain the end of last September. The temperatures during the day were mostly in the high 50s and low 60s. A light jacket was needed at night. Also, the entire interior of the cathedral was undergoing renovations and other than the alter and crypt of St. James, you could see almost nothing. Before deciding which city to visit, I’d check on the renovations’ progress.

See this recent, similar question for a few more replies, including my two cents: https://community.ricksteves.com/travel-forum/portugal/totally-torn.