Non-syncro boxes are usually harder wearing and have less parts so last longer. This all depends on the user double de-clutching of course. I have had three series 3 Landrovers with all-syncro boxes and my first car, a 2A, with syncro only on 3rd and 4th. Guess which one was the only one that I didn't wear out to the point gear changes were almost impossible? The syncro rings wear out and then you're in trouble. My 126 box has a bad change into 2nd because of the previous owners' bad driving habits, relying on the syncro to much.