I would say of all 3, the DTM X-4 is the most fragile. BUT that is because you have to use a smaller servo which has weaker gears.
Really, in all 3, the most fragile pieces are the servo gears. If you keep the body on your Mini-Z you should never have a problem. I have had my Mini-Z for over a year a, last week my cousin (younger) did manage to break a gear

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In the DTM SP, there really isn't anything to break. I did get hit by a 1:10 scale touring car and it bent the rear axile. No big deal, I think they are like 2 bucks!!!
As fare as drivetrain on the X4, I have never broke a thing.