The distinction for me between shuttle and straight gear is that you get way more reverse speeds with a shuttle, rather than one or two, as most straight gear tractors give you. They are also usually fully synchronized such that you don't have to come to a complete stop to switch from forward to reverse. My shuttle has 8 forward and 8 reverse speeds. The reverses are about 80% of the corresponding gear/range of the forward speed. Makes backing up real nice. Full hydraulic shuttles generally can be shifted without clutching. But, these types are generally only found on more expensive, "larger" tractors. Synchros are found on most modern mechanical shuttles, but rarely hydraulic, clutchless shifting.
As far as which is more durable, I don't think there is that much difference. You might have more HST defects off the factory floor because really HST is more complicated than manual. The complexity is just hidden from the user inside the tranny. I love HST, but am happy with the shuttle too.