Go for 120 round mags. I run a NGRS SCAR H, (90 rounds mags) which I have thought about switching the magazines to 30 rounds.
But…..
Your in a woodland, terrain gets in the way of “that shot”
Rentals with hi cap mags spraying around!
Even with six mags only with 90 rounds, I very infrequently go though three mags.
You will come across a game when you will run 30 rounds and think, “If only I had more bb’s in that mag, would of got that person/objective”.
Each to their own.