I use higromite and love it......I HATE hydroton....used them both. Higromite hands down BUT it does have its own issues. Among them: Heavy....they are ROCKS and they weigh the same as rocks. Also algae loves higromite and grows on it like mad so you need to cover it. It is resuable and plants LOVE all the silica imo and there is less fert build up in the medium because of their more solid nature. They are not quite as "airy" as hydroton but I definitely prefer to hydroton. (much easier to clean and reuse too.)

Hydroton pros:

Lightweight
algae won't really grow on it
Resusable
airy

Cons:

super amounts of fert build up due to airy nature
not stable.....rolls around like coco puffs even with a plant in it so no real stability
crappy ph issues so needs a LOT of pretreatment
IS resusable but a REAL PAIN IN THE ASS to clean.....again because of its airy nature.