In the US? Neither, FFS. It’s all McDojos as far as the eye can see…

Take her to both (or maybe even more) classes and see what she likes best. I trained TaeKwon-Do when I was a kid, it was great exercise and my teacher was very military like so it taught me some discipline, plus he really leaned into teaching the 5 Tenets of TaeKwon-Do. As a teenager I also trained some Karate with a random guy who ran a sushi place and had very Mr. Miyagi vibes, he was also big into zen philosophy. I’ve trained lots of other stuff as well, but at the end of the day it’s all to do with what you want out of it, if it’s just to have fun whatever she prefers will be best, if it’s to teach her self defense possibly Krav Maga or MMA will be the best options, Krav Maga is very focused on self-defense with lots of stuff that would be illegal in most other martial arts, and MMA is the most well rounded rules bound martial arts there is, it essentially borrows from all of the others to train you into being good at all.

I did both TKD and Ishin Ryu Karate as a kid: I think either or both would be fine really.

More important is the teacher and how the kid gets along with them/the class.

I would also stress that neither are ideal for practical self defense, so if that ever becomes the goal, look at something like Muay Thai, boxing, or Jiu Jitsu.

At that age it is all about the trainer(s). Personality, positive attitude, empathy etc.

The kind of sport is not important at all. You can change easily after a half year or a year if it was seriously the wrong one (but even then I predict that it is more because of the trainer than the sport itself)

At six years old it’s about the gym, coaches, and culture rather than the specific martial art. Find one you like and your kid is enthusiastic to keep attending. As someone who worked at a gym for a while and kinda know the various business strategies, make sure you factor in costs of testing, tournaments, equipment, etc. and not just monthly membership. Some have big monthly rates. Some are low but charge for every little thing.

If a kid really gets into martial arts and wants it to be their Main Hobby, it’s good for them to try various arts anyway.

I had a very good experience with Tae Kwon Do as a youth, taught me skills I still use daily. Make sure you find a gym that focuses on teaching, not just moving kids through the belts.

My 6yo staryed Tai Kwon Do and still does 2y later. The nice thing about it is that there are different disciplines. Style or fighting. So they find something they like. He also said that he needs it now. Although after a year he wanted to quit. He stuck with it and now he misses it if he misses training. Don’t know karate. For what it’s worth.

It depends a lot by the style and the trainer. I was training Go-Ju style Karate for 9 years and had a positive experience, while I had a negative one with a different style years before. All martial arts has a lot of different styles that can be very different between them. In my opinion the important thing is to find a school that doesn’t have too much people and where she can be actively followed by the trainer. Then, it also depends if you are looking for a sport/competitive experience or something more traditional. In my case we were training it as a martial art and went rarely to competitions.

Is there a way to try out both and see which one the child prefers? Maybe the clubs have some open door events in the autumn?

I did karate and judo around that age. It was mostly about games and having fun. Not really about the techniques etc. Definitely not about self defense or things adults relate these kind of sports to.