The design of both lures allow an angler to penetrate cover if needed and/or also to hop/drag them along the bottom. Variations of each also allow an angler to swim the baits as well.
Both lure categories have a deep history of success in the hands of some of the best professional anglers in the world. So given their past successes and their adaptability to multiple fishing scenarios, how does an angler know when to tie on a jig versus a worm?
Jig or Worm? Use What You Have Faith In
First, what are you most confident with, the jig or the worm? For me it's the jig because I simply have won more money in tournaments with it than any other lure. In a way it becomes self-fulfilling. Because I throw a jig a lot, I'm going to discover more patterns with it and that often translates into tournament winnings, and thus creating even more confidence in the jig! The same can be said by other pros whose confidence choice would be the worm.
Second, look at the forage base in the fishery and figure out what the bass are eating. When bass are eating small fish such as shad, shiners, bream, smelt, and so on; then I'm much more inclined to chuck a worm of some kind. This factor seems to be a bit more important (I don't really know why) when targeting smallmouths and spotted bass as compared to largemouths.
Jigs are often thought of as imitators of crawdads. Therefore, if evidence, such as regurgitated crawdad parts, suggests that the bass are feeding heavily on crawdads, then I'll choose a jig. However, these statements are certainly not absolutes! For instance, swimming a white jig can be very successful when the bass are feeding heavily on shad. Such adaptability is a testament to both lure categories.
Third, try to gauge the feeding mode of the bass. Really aggressive bass (I'm talking about the ones that are just waiting, cocked, and ready to explode on anything that moves), will take either a jig or worm. Unfortunately, bass are very seldom in such a state of hyper-aggressiveness, but when they are, the jig usually is my first choice. In part, that's because of the confidence thing, but also because I can really bulk this lure up and increase my chances of drawing bites from bigger bass.
Conversely, does that mean when the fish are off I prefer worms? The answer is yes. Some of the common reasons for bass to be off are bad cold fronts, post-spawn blues and fishing pressure. Worms are more subtle, offering slimmer profiles, and usually better disguised coloration. Finicky bass still fall for them while ignoring a jig. I've often changed my day of fishing under tough conditions by setting down the jig and picking up a worm.
Are There Big Ones Around?
Fourth, consider the number of big bass available in the fishery. I tend to favor the jig when I'm fishing a lake or river known to produce lots of quality fish. That's because overall, jigs produce bites from bigger bass as compared to worms. With a jig, you may be limiting yourself on the numbers of bass caught, but the size will be there. Worms often have the potential to catch more bass, but many will be little guys that are of no help to win a tournament. But if I'm on a lake that has a marginal population of bass, and limited bigger bass, then I'll prefer a worm because any keeper is a good keeper in this scenario.
Remember, that what I've presented here are guidelines. There are very few absolutes in fishing, particularly with this discussion.
The best advice and strategy is to rig one fishing rod with a jig and another with a worm. Being flexible and adaptable until patterns are established is what successful bass fishing is all about!