Kronos - By far has bigger DC issues than Nos ever did. Nos DC'd maybe once per hour or so at max, on Kronos the DC issue is constant - Since the Kronos server don't have the same OP botting autotesting, as a result of fixing some of the stuff, a fix could cause another bug somewhere and so on. For example, as of today, due to some fix, Verog hasn't been able to be spawned for the quest in barrens. - Smaller population, iirc it has about 3k peak while Nost has 6k. And im pretty sure that most people playing on Kronos are Kronos fanboys who were waiting for it, and not people who migrated from Nost. - P2W
Nost - There are some minor bugs here as well, like the 3 quests in desolace don't work, few quests in Feralas missing, and what not. When I was doing Uldaman, and we failed at the waves, we were unable to continue the run after running back. - Seems to be run by dedicated fans, eg. people who want no donations for their work.
From high end, I have not tried 60 yet since my Pala is 49 on Nost. If there are some scripting issues in those high end raids, it could become an issue. But from what I've seen, Nost is a much more stable server, able to support the population it has better, even before they got the new hardware. As an additional bonus, they have automated bot testing whenever new patch arrives to ensure all abilities work correctly, as well as 16 developers; Kronos has 3? developers on the project IIRC. I tried leveling a priest on Kronos, it's 15 atm, and I just couldn't bear with the CONSTANT D/C's and constant rollbacks, and I don't think the Kronos server will ever be able to handle that.
All in all, for now I'd concider the long-term capacity of Nost much higher, but who knows - the people running it might just get bored of it eventually down the line and dismantle it. I think that's kind of the nature of servers run by fans. Wherea's Kronos will likely stick around for longer due to the P2W aspect of it.
TL;DR; Kronos isn't "amazing" as it is portrayed to be. I have not much experience from private servers in the past, but the leveling experience on Nost is moother due to less DC's. If Kronos can't fix their DC issues soon though, I think that could be a problem.