If you are so unsured, why not just use the 70mt then use the 90mt for update. My bro was using the 70mt then updated to the 90mt. I have used all the discs and it ran the same as my bro. Like SWDW said, you aren't missing anything from the 80mt.
I have used the 80MT for awhile before purchasing the 90MT from SWDW. Like he said,the 90MT ran much smoother. Start-Up screen loads much quicker than the 80MT.