SMG and fire.
Story reasons: The SMG (both ODST and halo 3 ones) is a small and easy to pick up making it a good survival weapon, and seeing as were on a new planet surviving may be the first priority. Fire was are birth right as much as tech was the forerunners. We as humans have masterd fire and was are first true form of weapon. Also Th ship we were on sould have known it was going to fight the flood so im guessing a few flame throwers should be on board.
MM reasons:Its funner to use then a assult rifle and you can dual wield, and its just fun to see some one catch on fire in a game.
Also is there a way we could keep both weapons when we switch. I mean really. Having to drop a weapon to throw a grenade makes you waste time to pick it back up. Its much faster to hook it to the belt grab the grenade and throw and un hook the gun then to completely drop it and halve to pick the thing up. plus Having 3 weapons on the body instead of 2 is much funner.