Welcome, brother. ÂÂ

I'll throw my 2 cents in for Marines, but someone else will have to take stab at the Guard for your mate.
Space MarinesYou've got a single Tactical detachment with no transport (Rhinos), so you have a lot of "growth potential" there to get to 2,000pts.
Marines strength is their mobility and their flexibility. Their weakness is their limited numbers. To use them correctly, you need to get them into the fight before your opponent has a chance to whittle them down and reduce their effectiveness. The best way to do that that I've found is with Thunderhawk gunships, and Landing Craft laden with infantry and armour.
Here's what I would target with your purchases to bit the magic 2k mark (itals means you already have it):
Tactical Detachment (6 stands) with Supreme Commander upgrade (400pts)Devastator Detachment (4 stands) with Librarian (300pts)
Devastator Detachment (4 stands) with Librarian (300pts)
Thunderhawk gunship (200pts)
Assault Detachment (4 stands) with Chaplain (225pts)
Devastator Detachment (4 stands) with Librarian (300pts)
Thunderhawk gunship (200pts)
The objective with this list would be to smash your opponent's most expensive formation early (for the BTS victory condition) to put him off his game, and then seize his blitz objective (for the Blitzkrieg victory condition). The central tenet of this list is, to reference the Rogue Trader rulebook: "Loss is acceptable. Failure is not."

The weakness of this list is the low number of activations. The difficulty would be in isolating his BTS formation for your air assault--not to mention holding him off his objectives/victory conditions.
The advantage of this list is that it plays to the Marines' strengths, and it would be relatively cheap (good bang for buck).
For this list, you would need to purchase the following:
two Space Marines Battle Company box sets
two Thunderhawks
one Marines character pack
Expanding upon this list later, you could add a Reaver titan, Land Speeders, Predators, Land Raiders, a Landing Craft, and Terminators.
Again...just my 2 cents. ÂÂ
