All three: Quality (most YSS can actually be overhauled or at least re-sealed), performance (at least overhere you can have them built to spec) and price (Hagons are for what they are, terribly overpriced, only being approx. 100 Euros cheaper than some proper midrange YSS).
Also I tried Hagons and YSS on my XT500 and the Hagons were absolute rubbish, the YSS were built for my weight. I admit, I later on ended up with Bilsteins on my XT500, because I needed longer shocks, so I can't say much for the longevity of the YSS.