Covenant:
>300+ Ships in orbit around the planet (Frigates, Corvettes, Super Carriers, etc)
>Innumerable Troop strength with all species accounted for
>Assaulting numerous points of strategic value at once
>Like on Reach, they have the element of surprise, the Space Marines do not realize they are being attacked until the Covenant have a significant military presence planet side.

Space Marines
>Mix of Ultramarines, Blood Ravens, and Dark Angels
>spread across the planet, defending points of strategic value.
>have access to all vehicles, however they are in limited number
>3 grey knights
>No Titans
>No Primarchs
>A liberation fleet is on route to reinforce them, but the space Marines will have to hold out for 60 days until they arrive
>Exterminatus is possible, however the Space Marines will still have to wait the 60 days until the fleet arrives before it becomes an option.

The Covenant are after something that the Space Marines unknowingly have in there possession (forerunner tech, etc). The Covenant are also unaware that the Space Marines are even on the planet until first contact occurs.

Because these threads are always fun/interesting