Your priorities for IG depend on what kind of list you want. Heavy on the big guns or heavy on the numbers?
If you want an elite mechanized force, a few veteran squads in Chimeras or Valkyries is the go, flyers especially if you decide to go Elysian.
Or if you like the idea of massed infantry, start building platoons.
Either way, just remember not to overdo it. Both list options give you the choice of a lot of heavy weapon fire and the ability to throw a lot of blast templates down. Just keep in mind this is not always fun to play as or against.