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.
Yes the STX 12 and 15f are reliable watercraft. It's the new Ultra models that can be a craps shoot. I like Ernest have never ridden one but remember when they first came out in 2005 people were saying great things about them. If the price is equal I'd go for the Yamaha since it's a higher end watercraft (and faster). But if you can get a smoking deal on an STX Kawi you'll be happy too.