The initial radiation and heat wave is likely more intense because there's no ground to shield between you and the blast but, as someone else said, most radioac

The initial radiation and heat wave is likely more intense because there's no ground to shield between you and the blast but, as someone else said, most radioactivity is from induced activity in ground material sucked up in the blast, so from that perspective air bursts are much less "dirty" as the fission products rise into the stratosphere and are dispersed in the winds. Definitely it would be better than being 3.5mi downwind of a ground blast, but probably not compared to being upwind. The shock wave is probably also less damaging because it comes from straight above so it won't pick up objects and throw them at you.