Totally different hull. I had an X-9 and have ridden behind an X-14. From a slalom side, the X-9 had a bigger wake. Wakeboarding, the X-14 has a better lip and can be ballasted more. Is it worth the $10k? Your call.
The layout of the X-14 is much better for wakeboarding.
Let me know if you guys want to stop in and check some out. We have a couple X-9s and a couple X-14s laying around. FWIW I would totally agree with what Karl is saying.
Since your dad slaloms I think you guys would be happiest with the X-14 but the X-9 is pretty fun none the less. I was all set to take one an X-9 last 4th of July and then it sold on July 3rd!!!