I've taken TKD and Kung Fu and my children have also taken Karate. I don't think it really matters what you do as long as you do something. I agree with others to go to the class and observe and find the instructors, schedule, and pricing structure that works for you.
Other things I would think about asking...
- how frequent are tests and how quickly to the children move up. Tests usually cost more and it can add up. On the flip side, some children respond really well to frequent testing and like the fast progress rather than waiting.
- what kinds of weapons are taught and when. Some kids are very motivated to start working with weapons so if the school doesn't teach weapons, that may be a consideration. Alternatively you may not want your child using weapons.
- is board breaking part of the program. Some kids are way into this - some are intimidated.
ETA: and ask about sparring, how it's done, what pads are used, etc. My kung fu teacher had us adults spar unprotected and the small kids didn't really spar at all. In TKD and Karate - pads were used and it was very controlled.