Definitely bolt weapons, and a storm bolter. If you're tolerant for very strong cheddar cheese, make Ambidextrous and Two-Weapon Wielder (Ballistic) top priorities as soon as you hit Rank 2 and can afford them. I once ran the numbers on Astartes Storm bolters, and dual-wielding them has a higher damage output per round of firing than a Heavy Bolter on Full Auto. Normal Storm Bolters and normal Heavy Bolters scale roughly the same as their Astartes equivalents; even with the -10 penalty, you will annihilate pretty much anything you point your guns at with a decent investment in BS.