the main difference between slim one and the original one is that slim has a built-in network adoptor and orginal one does not have it.
the earliest one i-e 39000 series was made in japan so it was highly reliable while other series like 50000 and 70000 series are all made in china.
i have 70000 series slim one, 39000 series original big one made in japan is still expense than a slim one.
my friend has the original one and he didnt get a single problem with it.