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FWIW I just broke an XTR derailleur with a baby head hitting it from the bottom. It seemed to break easier than I expected. I replaced it with an SLX and can't tell the difference in performance. I have no reason to believe an XTR would be stronger than an SLX, just 3x as expensive. I broke 2 GX Derailleurs in 3 years and now broke 1 XTR in 2. Derailleurs I find to be one of the easier things to break on your bike. I don't see one brand being stronger than another, it just comes down to good...