[captain harlock]

The 6 spyder diffs are way easier to maintain and assemble. Moderately priced and light weight. The only problem is when you put the diff under heavy load in a heavy truck. The gear housing might break or get twisted. 8 spyders are simply made for extreme racing condition for better handling, extra durability, smooth operation and zero flexibility.
They're very expensive, complicated and quite heavey which consequently increase rotational mass and decrease acceleration a bit. They do look killer, though. :)