Airbag down pressure systems on planters have two primarly advantages over springs:

1) Much more uniform down force from the bottom to the top of the row unit travel range. Springs vary enormously in downforce transferred from top to bottom of the row unit vertical travel.

2) ease of adjustment on wider planters

In my mind item #1 alone is worth using air rather than springs unless your fields are flat as pool tables then it probably doesn't matter much either way.

jmho.

Jim at Dawn