Having had both manual transmissions with 80/90 gear lube and ATF, I can tell you the ATF is a lot easier to deal with in cold temps. I can also tell you the ATF can identify a seal in poor condition a lot faster then 80/90 will. I also believe the ATF lets you have noisier gear boxes on those hot long driving days then the 80/90 did, but that might be because the ATF trans has nearly 200K on it and I don't remember many 80/90 gear boxes that had much more then 120K miles on them.

I can also tell you the two lubes don't mix well, it would be good to choose one or the other, freshen the lube, and stick with your choice. Much of the "newer stuff" (mid 70s and newer) use ATF and the older stuff used 80/90. Unless your manual trans is newer then the 1990s, and you do mostly warn weather use, I would probably go with he 80/90. Newer stuff, ATF. Gene