Try buying a used automatic over 150,000 miles and change the fluid first thing, if you don't between their and 300,000 your going to have break and need replacement within that space of time, but at the same time the engine is 150,000 miles and if it's dirty enough replacing the fluid will destroy it easily.
Don't get me wrong I am not sh*tting on automatic transmissions here, if you buy a car brand new and religiously change the transmission fluid, it isn't a issue. But, if your buying a car used, a manual will have a longer shelf life.
In the entire debate of Manual vs. Automatic. Nothing trump's longevity, as you want your vehicles transmission to last above all else because if it fails they are so expensive to rebuild or replace you might as well by a new vehicle by that point. Manuals have a advantage of the clutch taking most of the wear over the transmission itself which increases reliability.