Stick with 3.1 if it runs alright for you, i only installed 3.9 because it was free. 3.5 & 3.9 look like someone went crazy on Aminet and downloaded a load of stuff to make it look pretty and a few system files updated here and there.
The only positive thing i can say about 3.5/3.9 is, it is better to install from scratch than 3.1.
