Well I feel that the added complexity of the eight speed automatic transmission would prove problematic over the long run. I want to keep my cruiser for five or six hundred thousand miles and I feel that not only would there probably more likely be an issue with the eight speed but if there is it would be a lot more costly to fix than a six-speed. Also the six speed has been out for a lot longer so Toyota has had time to perfect it.
Personally, I feel that five or six gears is enough for any vehicle. I like my vehicles mechanically simple. That's why I fell in love with this old school Toyota V8 engine. No gdi, no turbos. None of those tricks to try to squeeze out an extra two or three miles gallon. That's just me though.