Would like to visit both, but expect I will have time only to do one. Which would you recommend? Priorities: architecture, nature, walkability, photo ops....... . Would be in summer or early fall.
I've been to both. From a photography perspective I think Aigues Mortes may be best. I felt like Sete wasn't quite as distinctive (architecturally) as Aigues Mortes, though I was fortunate to discover a multi-site photography exhibition in Sete and really enjoyed wandering around the town, locating all the exhibition venues.
Be warned, though, that Aigues Mortes is very touristy. It is visited by both people interested in the walled medieval town and those wanting to see the nature reserve (which I didn't visit, but I believe it's known for flamingos). I think I stopped by the local tourist office to pick up a handout identifying some of the most important local buildings. Oh, one other thing: I vaguely recall warnings about mosquitoes in the marshy nature reserve; I didn't have any issues in the town, where I stayed overnight, but I'm lucky not to be particularly attractive to mosquitoes.
I remember the walled part of Aigues Mortes as being absolutely flat. Sete is somewhat hilly, as a careful look at some of these photos will show: https://www.bing.com/images/search?q=photos+sete+france&qpvt=photos+sete+france&form=IQFRML&first=1&tsc=ImageBasicHover
Aigues Mortes will probably be a more interesting, one day destination than would be Sète. Sète is well known as a fishing port, Mont St. Clair, seafood restaurants, and George Brassens, a well known French composer of popular music, his most popular song probaby being Les copains d'abord. It is basically a pleasant small town but not a compelling visit considering what one might otherwise see in the area.
Aigues Mortes was once a seaport, from where many of the Crusaders departed Europe. Both Aigues Mortes and Carcassonne are old walled cities but I find the former more authentic than the latter which rebuilt several times. There are tours in Aigues Mortes offered by the Office de Tourisme which is usually located just to the left as you enter the city´s main gate. Tours are available in English and you can be guided through the town, its long history explained, and generally includes a visit of the Tour de Constance, its old dungeon. Just outside the entrance to the main gate is l´Escale, a family run restaurant that has been there for many, years and offers typical French cuisine at bargain prices, always including a carafe of wine with their 12€ lunch. My guess is that l´Escale will survive Covid and still be there when you visit.
From Aigues Mortes there are sever tourist boats available which will take you along the Canal du Rhône a Sète and into the Camargue, the wet plain or grassland home to the famous white horses and extensive bird life to include flocks and flocks of flamingos. The Ornithological Park, about 20 minutes from Aigues Mortes, on the road to Stes Maries de la Mer is home to an amazing array of bird life which you can usually see up close. Flamingos are extremely shy birds and it is rarely possible to approach them.
Over the centuries, as the Mediterranean Sea receded, Aigues Mortes became landlocked. To continue its important position as a seaport, it was necessary to build a canal to the golf. Grau is an old French word for channel or canal and as it was the king who authorized and paid for the work, the canal, and the town at its seashore end, became known as the King´s Canal or in French le Grau du Roi. Le Grau du Roi, was up until recently, one of France´s primary fishing ports and is an interesting visit of its own. Aigues Mortes has many tourists and lots of Americans but little more than a mile away, the tourists never seen to make it to le Grau du Roi which is a vacation spot for the French and sometimes for the Dutch. I always found it odd but street signs here are in Dutch as well as French. Le Grau du Roi has a Kiddie Land (they call Babyland), putt putt golf, a large water park, and a noteworthy sea aquarium. Anyone traveling with small children would find Le Grau du Roi a vacation mecca. The beaches here have very gentle waves, perfect for little people. It´s a great place to avoid the English speaking tourist hordes. For dinning in le Grau du Roi, I recommend le Dauphin. If tellines are available, they should not be missed. This is a delicacy that, to my knowledge, has not been commercialized meaning they are only available in this region.
On the road from Aigues Mortes to le Grau du Roi is the Baleine Saltworks offering tours in English. Just next door is Listel winery that used to give tours in English but I cannot find confirm they are still open.
Overall, I believe Aigues Mortes can fill several days, just to see the basics of the town and surrounding area. You might check with the Office de Tourisme for more information.