I am also using the FlyFF v18 (really v17) posted. I guess it depends on how far you are into your projects and if you have fixed all the issues within the Flyff v18 by ketchup. I will say there is a suspicious reason ketchup releases a new "Base" every few months. And lately has become a money grab.
V15 tho is a for sure way to start fresh and build the server the way you want it. and fix any issues that arise along the way. You will know everything wrong with your source and it will be your own fault if its bugged in the end.
But, you will need to fix and secure any set of files you download no matter the source. If you are content with your knowledge you can make the worst files stable. just keep your eyes open.