Depends on your needs, do you intent to go to production any time soon? And do you have business users depending on that application? In that case definitely go with an already stable release like LTS; 9.24.
If you are developing for a few months and not releasing to production or don't have many users depending on this application, and you are still figuring out the scope of the app / releases; you could go for Mendix 10, But even then, the safe bet would to start with 9.24 and just upgrade when you need specific features from 10 or a MTS has been released.
Altough its 10 is a major release the last versions of 9 won't be much different from the first few releases of 10 in terms of capabilities and structure. I believe its more a PR / marketing thing to bundle and enable new features in a major “10” version. Most new features and code changes were iterative added and were already present in 9.18, 9.19, 9.20...etc. but some are still behind feature flags etc.
tl/dr; i would go with 9.24