They are both same grade of oil 90, but as it's a multigrade it's modified for give similar protection in colder weather so 75w gives a bit more than 80w.
I use 80w90 as I also live in the southeast so its not that cold in winter -5c maybe but in Scotland -15c thay may wish to go with 75w90.
There's lots of info on the www on gear oils, grades and operating temps.