the B&M uses your oem shifter housing with a new stainless steel machined lever and components in anodised blue aluminium, you get an option of two positions both which reduce the throw by moving the pivot point, you also get spacers to lift the housing an inch which will put the shifter closer to the steering wheel if you want to fit them.
The BC uses a completely new moulded housing and a lever with a slotted tag welded on to move the pivot, unless you get the cable perfectly aligned in the slot and done up tight enough not to move in the slot you could get mis shifts, which ultimately could damage your gearbox.
I have personally adjusted two BC shifters during trackdays as the bolt has come loose
for this reason i would pick the B&M every time.