It doesn't make too much of a difference, what order you go in.
Let's say a regular attacker takes 10% of the land in one hit, and an avian only 9% (due to -10%)
If you have a 1000 acre province and you attack with Avian then regular:
1000 - 90 = 910 - 91 = 819
OK, what happens if you attack regular, and then avian?
1000 - 100 = 900 - 81 = 819
See what I mean? NW and GBP does complicate this a little bit... but no need to overthink it in my opinion.
@magGainers: I've noticed ghetto noobs aren't that good at max gaining... how they pick their attacks. do you have any advice on that?