Guess what? I have both of them - 2012 12m Varial X and 2011 15m Edge. I rode them a lot with directional Sector and started to ride them on twin-tip. Although, edge is my light wind kite and varialx is my all round medium wind kite. Eventhough my comparaison is not size vs size I still think I can comment on them side by side :
First of all, I love them both, they are somewhat simillar kites, they don't like when the wind dies, they are heavy, and high aspect so you have to move them if the wind has holes and you're at the beach and heading out and don't want the kite to fall. They are not the most user friendly, do it all, kid of kites but are amazing at what they do.
VarialX got good reviews saying that you always know where the kite is, and it's true, you just feel where it is around the window, coming out of downloop you don't even need to look at it, you just feel it. It's got some bar pressure, I'd say 15m edge is lighter on the bar, then 12m varial for me, but not tiresome at any point.
Both go well upwind, but edge is better. Both are good jumpers, I'd say simillar height and glide, even though I can't compare same sizes. What i can say that in light wind the 15m can lift you up wayyy high and you just glide down.
Ozone got awesome built quality for the kite, but airush got better bar, nicer designed and safer ,IMO, quickrelease, working swivel and adjustable bar length.
In conclussion i'd say, both are amazing kites, that like to be flown somewhat to get the maximum from it where VarialX will shine a little more as do it all kite kind of kite, even though it does not pretend to be in all arounder as there it's cousin Lithium, while edge is even more racier, even though VarialX can get amazingly fast on a directional, I did my fastest runs ever on it with sector . Edge can race downwind too, it's really a racing kite that jumps and glides.
Can't wait to try the 2014 VarialX though as it get's totally redesigned and loses 2 struts.
If you go for Airush, go for 2013 especially for the bar redesign. Kite got improved too. Edge seems to be same for 2011-2012, and got little tweaked for 2013, same bar though.
let us know what you choose!