I have used a couple of different designations in my games;
Marines
Orbital Landing Force
Drop Troops/Drop Infantry
Mobile Infantry
Bounce Infantry (Old School Renegade Legion!)
Assault Legion
I think it kind of boils down to how you see these formations working. If its an amphibious analog, they tend to be based around Marine formations. So, heavy infantry and infantry support components, but light on tanks/vehicles. For that matter, if you filed them as armored infantry, you have instant Mobile Infantry/Space Marines.
If you see them more as planetary assault troops, i.e. large formations of tanks, support, and troops, then you could argue they are more like army formations. The old FASA board game postulated that when the tanks themselves could drop independently from orbit, Marines become superfluous and go back to being shipboard formations.
I think of lot of this just comes down to how you see the "lore" of your space empire working out.