I can't speak for the BMC - never ridden one - but I picked up the Flash 3 a month ago. STAY AWAY. The KMC chain, FSA crank/gears, and Sram derailleur do not play well together, and chain suck is common. Even if that was a non issue, the frame is limp as a noodle. A car pulled out in front of me, and I grabbed the brakes pretty hard. The frame started flexing at the head tube, then springing back like a jackhammer. HARD. I came close to coming off the bike, and ended up running into a bush instead of hitting the car.
I had the LBS talk to the Cannondale rep about it, and he said that is normal for a 29" carbon frame. NORMAL? I got a FELT Nine Elite to replace the Cannondale. It is a 29" carbon frame, designed as a XC rig, and it is solid as a rock! The only way that is normal is if you have an incompetent designer working for you.
Go with anything but Cannondale. There is no way I would trust my life to that thing, and the attitude I got from the company rep was horrible. He basically said that it was my fault for even thinking that Cannondale may not be perfect.