Well, just on the surface analysis, a six-speed is less busy, it makes fewer shifts than a eight-speed. So, if a transmission has an average "X" number of shifts in its life, a vehicle that makes additional shifts may not last as long, all things (quality of engineering and manufacturing, driving habits, maintenance, etc.) being equal (a problematic assumption), then a six-speed transmission should last longer. Of course, the rationale for the additional gears is improved gas mileage.

If you seek comparisons of various generations and years of Camrys, I'd refer you over to this YouTuber: thecarcarenut. 

He's a Toyota technician who does comprehensive year/generational reviews of Camrys, Avalons, Lexus, Tacomas, Tundras, etc, You're likely to get a more detailed picture, there, given how well-structured his presentations have been. Look under the Camry "Buying Guide" for more information.(And, AMD is blunt in his assessments of what Toyota has done well, and not so well, in each generation and year):

https://www.youtube.com/c/TheCarCareNut/playlists

8 speed puts less stress on speed and 6 speed is more lag but quicker acceleration, it might be quicker. I never had one, but I would buy an 8 speed, they suppose to be on the luxury end. They both reliable as long as you dont drive them hard, just drive it like a normal car.