Both should run fine.
But keep in mind, that even if you are running on JDK9, you wont be able to use jigsaw modularity in your deployed application.
JDK 9 / 10 support in WildFly is currently limited to running in classpath mode.
But in any case I would go for JDK9 as it bring so many new features and performance enhancements.
If you find any issues running on JDK 9/10 please let us know, but as of now, don't know of any big ones.
It has a problem when using JDK9. You can see this post:
And this issue: