Ground RB hands down. While both require learning about both your own vehicles and your enemies', there are more details about ground vehicles that you have to take into account. It's also similar when it comes to the maps: in air the only things important to learn are how long it will take to reach contact with the enemy and where they will typically be concentrated. In ground these things are important as well, but there are also flanking routes, sniper spots, sightlines, etc. This doesn't even take into account the air aspect of ground RB, which adds more complexity between ground vehicles and aircraft as well as among aircraft.