I don't really know.
Both of them have pros an cons.
The XT Disk is only available with the
Shimano dual Controll Levers.
I don't like these levers for freeriding, but for XC they are OK, I think.
The power is not that enormous, but enough for XC.
I don't have the "scream" problem when it's dry, only when its wet outside.
The pads are not the best ones, but if you don't brake without any pause, this doesn't matter.
The Hfx 9 (I haven't already testet) has the "on-off" principle, as many people say, which means either it stops hard or not at all.
It's hard to dose the power.
The power all in all is better than the XT-Brake, as I heard, but as I told you, I haven't already tested.
If you are a seasoned (disk brake) rider, I would recommend you the HFX brake, If not, I would take the XT, but you must like the system.
All in all, I wouldn't take any of those personally.
Tip: The K18 or K24 Formula Brake. I love the design and the power is enormous, but also a bit hard to dose.
So long,
Maddin
PS: I'm only a pupil, so please excuse my language mistakes, if I did some, but I liked you asking here.
Don't you have MTB forums there in Denmark or where you come from?
It might be easier for you to get lots of different answers and more experience with the Brakes.