It's been a couple of years since I saw several Flamenco performances in Sevilla. And I've only been to one of the venues that you mention (Teatro Triana) so I can't compare. But I found this guide that includes both.
https://www.espanaguide.com/seville/where-to-see-flamenco-in-seville/
From what I've read, both of the venues you mention are what I would call theater style Flamenco. Certainly at Teatro Triana, you are in a small theater style setting, with rows of comfy theater chairs. Teatro Triana seats 100+ guests. (The article says 126.)
The article says Tablao Las Setas seats 140, referring to the venue as intimate not cramped.
In my experience, the intimate venues seat perhaps 40 guests, maybe 60 if they have an upstairs. The chairs have often been wooden and the venue may feel small, perhaps cramped. The tradeoff is that you are generally very close to the performers. A foot or two away sometimes. You can see every move of the dancer's feet and hands. You can feel the movement of their stamping shoes. This is what I think of as an intimate venue.
In a theater, you are watching a performance from more of a distance.
Now, I would be very interested in learning more about Tablao Las Setas, to understand how they create an "intimate" experience for 140 people. If I were to go back to Sevilla, I would give it a try.
Generally, I prefer venues that are less theater style and where guests are nearer the stage, at minimal distance from the performers. Perhaps reading the guide with this distinction in mind will help you choose the venue that might suit your tastes.