I've been told in Aus 92 and 98 octane fuel has 10% ethanol content, 95 octane never contains ethanol. As a result I always run 95 octane with stihl ultra oil at 40:1.
As you would know the 6400 is very respectable it does the job. In 7900 form it flat out rips and you can be more heavy handed with the saw. I have only run both with a 20 inch bar, I would imagine it would make a big difference with a bigger bar. I would say if this is going to be your biggest saw then go to 79cc. If you plan on getting a bigger saw in the future then leave it as is and save the funds for the bigger saw.
As you would know the 6400 is very respectable it does the job. In 7900 form it flat out rips and you can be more heavy handed with the saw. I have only run both with a 20 inch bar, I would imagine it would make a big difference with a bigger bar. I would say if this is going to be your biggest saw then go to 79cc. If you plan on getting a bigger saw in the future then leave it as is and save the funds for the bigger saw.