The Flux 27.5 is still surprisingly light, so rides okay for XC too.
I prefer the feel of the Czar as it’s just so much sharper; wheres the Flux feels more a bit more leaden. There a few factors so it makes it difficult to nail it down why. But I built my Flux fairly light but equally I’ve ridden it in the Alps etc.
The Flux is the bike I take to the Peak; ride mainly in the winter and generally put on my roof rack. The Flux is a long bike on the top-tube so feels bigger.
The Flux feels more of an out-going model – you can’t help but enjoy the benefits of the Carbon Czar but then do you want to give it the same stick in the hills. However for the Kielder 101 – the Czar was the right choice.
I do hear of people “knarling” the Czar up which is great but I don’t think that was it’s original purpose.