I'll agree with the other guys here, the Tech 8 is a bit nicer boot, feels a bit more supportive. I'm not sure if it's $75 better than the Tech 6.. If the 6 fits your budget better there's nothing wrong with it. If you've got your sights on the Tech 8 you won't be disappointed.
I wore tech 6's for about 2.5 yrs and loved them. Recently I acquired some tech 8's and while I have only got out with them once I can tell you they are different. The first thing I noticed (very important to me) was that they were way more snug than my 6's ever were. My foot size falls inbetween whole boot sizes so I was forced in both cases to go with the larger size. The 8's fit me like a glove while the 6's always felt a bit loose even with the bulkiest socks I could find. It appears to me that ankle support is much better also.
A bit more info, the 8's were hard to get used to. They didn't seem to fit properly for the first 3 or 4 rides, but once they broke in they felt like heaven. They've spoiled me and I'll definitely get them again once these wear out but like I said previously there's really nothing wrong with the 6's either.
The 8's are for sure worth the extra $$$ I also wore 6's for about two years and they were as comfortable as cheap dress shoes. Now I've got 8's and that inner foot bootie kicks ass!!! There's actually some padding and it's comfortable :cool: In addition there's a metal plate to protect your achilies (sp?) heel, can't remember but I think the 6's are just plastic.