IG/WH get the Vulture as a Land Unit at Flight (with a higher attack value), then get the Valkyrie at Heavy Fliers as a transport (with a lower attack value)
While the SM/DH dont get the Vulture at Flight, they get the Thunderhawk at Heavy Fliers which is both a bomber/"gunship" and a transport
Keep in mind the SM/DH get the Drop Pods, which are dedicated transport units, which come earlier than both the Thunderhawk and the Valkyrie