You don't hear many stories of people breaking either transmission, so it would seem the answer is, "more than anyone has bothered to put in their 135i."
A lot of this has to do with drivetrain slop (dual mass flywheel, driveshaft isolator, bushings, etc), tires, and abuse. I wouldn't worry too much until you are running on slicks with a solid flywheel, unsprung clutch, and beating the hell out of it.
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2009 BMW 135i 6MT Sport, AFE intake, Cobb AP, Apex 18x8.5+9.5, 255/275 PSS
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