"If you study military history since the invention of the catridge-bearing rifle, the dedicated fighting knife/sword has faded into disuse for good reason. Even during the American Civil War, soldiers often left for war carrying enormous bowie knifes and dirks which they soon ditched. They just aren't worth the weight that could be used as extra ammo, extra rations, etc. MY advice? Get a decent quality sheath knife that doesn't weigh too much AND that can be used as a utility knife. Don'ty spend too much on it, as it IS going to get the shytte beat out of it doing chores in a combat zone, you may have to ditch it, and it isn't at all unlikely that it may be stolen...perhaps by one of your own comrades. You don't want to lose hundreds of dollars. Also get yourself a good utility pocket knife of the SAK type. Get a good one, not one of the Chinese copies. The openers, little scissors, etc. are worth their weight in gold. What I'm telling you to get is a good working combo that is good quality but inexpensive enough to replace as often as necessary.. You can still stab somebody with the sheath knife, if it comes to that, but you NEED something that is worth the effort of carrying it around the other 99.9999999999999999999% of the time." Golgo-13
A lot of good points but these psuedo statistics are based on military persons who got re-supplied somewhat regularly. As an individual with limited resources and supplies, I would put a little more into it. When you run out of ammo and have nothing else left, then that knife is worth a hell of a lot more than rations. I spent a lot of time in limited resource areas (living off the land) and the knife is truly your primary tool that gives you the edge, and if taken care of will last you almost a lifetime.
Get one that is strong enough to use for utility and still effective enough for doing the primary job. But in conjunction with what Golgo says, I'd get a good quality multi-tool for the pocket. You will need that almost more than anything else.