It depends on the situation. The flamethrower is very limited in ammo reserves, and while it does more flame damage, flame damage in itself is not always the best option, especially when taking out larger Specimens. I would say just gauge the situation. However, if you're looking to level faster, my suggestion above is about as fast as you can legitimately do it: DeathBasinSands, Mac10 and Faked Players. I went from level 3 to level 5 in one afternoon, just by using a semi-auto Mac10.
Plus, it really teaches you the benefits of which weapons to use, when to use them, and why.