Seville 5-star hotel: Alfonso Rey XIII or Colon Gran Melia?

We have stayed in both and you can’t go wrong! Alfonso XIII has very extensive use of Moorish tiles including in rooms and bathrooms; Gran Colon does not. That is the main difference that I can remember. I liked the Gran Colon better, enjoyed it more but for being over the top, Alfonso XIII has the edge.

Kimberly, both are excellent hotels. The Virtuoso consortium ranks both among their BEST OF THE BEST hotels in the world.  
 As mentioned by Suki, they are a contrast in styles but you won't be disappointed by either. For a first-timer, I'd also lean towards the Hotel Alfonso XIII. It's also a member of the Marriott Luxury Collection, so you would earn BONVOY points for your stay.

I would recommend booking this stay through a STARS  or Virtuoso TA as you can get additional amenities, perks, and even non-published deals (sometimes). The current amenity list for a Virtuoso traveler would include: 

• Upgrade on arrival, subject to availability
• Daily buffet breakfast, for two guests per bedroom
• A complimentary lunch or dinner for up to two people per room (menu of the day) once during stay, excluding alcohol, taxes, and gratuities
• Early check-in/late check-out, subject to availability
• Complimentary Wi-Fi

The Luxury Marriott Amenity program is called STARS! The STARS program includes all of the above, a welcome gift, and an on-the-ground assistant geared to assist STARS clients. (Quite helpful in these COVID days!) . You would have to book through a STARS TA to get the STARS amenities and STARS rates.

At the Hotel Alfonso XIII, you can join a daily tour of the property for a closer look at its history, art, and architecture; the tour even includes a small wine tasting. Because of the history, unique features, excellent service, and additional STARS amenities, I tend to lean towards the Alfonso XIII. 

Thank you all so much! Valuable information that one can't find in a travel book. So glad I posted!