There is no comparison between an HX and an XP. As an HX owner, I shall provide my biased opinion.
An XP has higher top speed and is a little more stable than an HX. XP is also a common platform for performance modifications. And it has some basic storage and instrumentation.
The HX on the other hand doesn't sound like a good deal at first. It has an 85hp 720cc motor unlike the 110hp 800cc motor in the XP. There is no instrumentation (no gas gauge, speedometer, hour meter). And it has NO easily accessible storage for keys, drinks, etc.
But here is what is VERY special about the HX. It has a very deep-V hull and has the narrowest beam (width) of all the Seadoos ever made. It also had the first suspension seat in the industry at the time (until the next-gen XP in 1998).
It really feels like a motorcycle on water. Riders familiar with standup jetskis or the Yamaha Waveblaster will have an easier time adapting to riding an HX. It's unbelievable tippy nature at low speeds or engine off is very difficult to put up with. But that is exactly why the handling on an HX at speed is truly amazing - tight holeshots, deep carving turns and "throwability" make the HX exciting to ride.
If you fall off an HX, it is really hard to climb on from the back. It requires powerboarding from the side (think trying to jump onto a horse from the side while it is running. I highly recommend training any new rider on the deep water start BEFORE letting them go running around on their own.
So if you already have a Seadoo you are comfortable with - one that is a little easier to ride for beginners and is more practical for longer outings, then an HX is a great accompanying boat. It allows you to trade out riders when they get a little worn out on the HX.
For other readers of this post, if you are first time buyer that doesn't exactly know what you want, do not get an HX without test riding it first. It is NOT for beginners and is actually pretty frustrating if you prefer a leisurely ride.
That said, the HX is a thrilling ride. We just came from a trip where people would ride around for hours on a 3-seater 96 GTX. But after 30 minutes on an HX, they feel like they got a workout!
