Here is my 2 cents. Keep in mind that I haven't ridden the Kaw. In most aspects I think those 2 skis are pretty well matched. Speed will be close, with the newer FX HO probably 1-2 mph faster (160 HP motor on the Kaw and around 180 on the FX HO). Fuel consumpsion will be similar (Kaws never great on fuel even with smaller engine). Ride may be a little better on the Kaw in rough water, but close. You will get more bells and whistles on the FX HO. The FX HO is a bigger ski, but weight is similar. Both have the same storage capacity, but FX HO carries 2 more gallons of fuel. Reliability goes to the FX HO, but I believe that of all their models the 15F is Kaws most reliable.
I'd go with the Yamaha for reliability and reputation factor, but it would be a tough choice.