XXX Kinwald is old, but good. XXX BK2 should be avoided, not good (they knackered handling with the low CG gearbox thing on it).
I just swapped my Kinwald for a CR and haven't looked back - belting car IMHO. XXX CR is great for me.
I have encountered a lot of anti-Losi sentiment in my time and it seems unfounded to me. I have never driven more competitively than I do now with my CR and it's all about setting your car up properly and using good tyres. Do this and it will handle well, and the rest is up to your thumbs.
I'd be mindful about buying into the hype concerning the X6. Sure it is a good car (all these drivers can't be wrong can they?), but it's expensive (certainly more so than a good 2nd hand CR) and you'll just end up putting Losi shocks and other hop-ups on it anyway to make it as competitive as everyone else!