I’ve got an 09 575 with carbon rear end and I can’t say I’ve noticed any flexing with it at all, and this includes a day at Cwmcarn on the downhill run. It feels just right to me, and I still think it looks the nuts too. On top of that, it’s an anodised frame and the paintwork is as hard as nails. Still looks pretty new after a couple of years or mis-treating it. Mind you, what suits one person won’t suit another so maybe you’ll only decide after riding both. As for the rear end snapping I’m sure I remember a similar thread on Yetifan where it was revealed that they’d had more failed alloy rears than carbon ones. I think everyone worries that carbon’s too fragile but it seems to be the way all the high end bikes are going these days so it seems unlikely.