Imperium would win against terrans hands down IMO. Imperium has a much larger force if you count IG, SoB, Astartes, and the Inquisition. The Imperium also has a lot more technology at its fingertips I.E. Land Raiders, Predators, Titans, Battleships, Battle Barges etc. Space Marines are also a lot more hardy then Terran marines. Astartes have much better armour, are better trained, and have wider choice of weaponry then the Terrans. Remember, Terran marines lifespan is about 10 seconds on the battle field, while some Astartes live for thousands of years. So all in all my money is on the Imperium.
"Blood cries out to blood they say, so let us spill their blood so we can hear it all the clearer."