The weapons in PS2 are balanced and designed to be sideways upgrades; some are more effective that others in particular situations. The default guns are good all-rounders. LMG's have a large magazine and carbines are better at range. Each weapon will have it's own characteristics such as kick direction, full auto effectiveness and stopping power so you should always trial a weapon before deciding to purchase it.
For Heavy Assault there is one caveat; the rocket launchers. Personally I spent some station cash for the guided vehicle launcher and it has had a big impact on my play-style.
Best Answer
The LA80 has less bullet drop, which means it is easier to score long range headshots.