I personally feel that a well-presented worm will out fish a plastic worm in most situations. This difference in effectiveness is most apparent when fishing is hard, maybe due to high or low water temperatures. With that said, presentation is still key, a well-presented plastic worm will likely be grabbed before a badly presented natural.
Now, like all things fishing. There are always exceptions. When the trout are actively hunting and harassing baitfish or larger prey. I find an artificial worm works better because it can be cast out and retrieved like a lure.
Another situation where a plastic worm works better is when the trout are refusing to feed. Then a large plastic worm can sometimes trigger a strike out of aggression.
Different brings different results. Real vs plastic worm
Most people fish plastic worms and real worms differently.
When fishing for trout with a real worm. Most fishermen will cast out a worm and let it float just below the surface. They will let the worm float in the current, often beneath a bobber, and will wait for the trout to come to it.
Sometimes, they will also cast it upstream, then allow the current to carry the worm down and over the waiting trout. Just like presenting a nymph. Fishing with real worms can be a very relaxing style of fishing.
Now. It is possible to fish soft plastic worms in exactly the same way. Just float them downstream below a bobber. When the trout are hungry enough, it can be effective. Although, I, and most fishermen prefer to fish plastic worms with more action.
I fish plastic worms more like a lure. Cast them out then slowly retrieve them in with the occasional jerk or twitch of the rod to impart some action. When river fishing, I cast them out into the current, and let it swing downstream, keeping just a little tension on the light to feel takes. I then like to retrieve the worm in just slightly faster than the flow of the river.
So while it is possible to fish both types of worm either way. I personally prefer to fish real worms like bait and plastic worms like a lure. Despite a broadly similar appearance, they work best in quite different styles of fishing.
Advantages of real worms
They are real, so they perfectly represent the real thing.
A real wiggling worm is the real deal. They look exactly how they should. That means the trout are less likely to be suspicious of them which increases the chance of hooking up.
Live worms, wiggle, and move through the water.
Worms are living creatures and will continue to squirm around for quite some time even with a hook in them.
These slight vibrations not only attract trout but can even trigger fish to strike. Contrast this to a plastic worm, which just lays there unless actively retrieved.
Live worms likely give off a scent.
Real worms smell exactly how they should. They do not smell like plastic or garlic scented plastic.
While I do not believe scent is a major factor when trout fishing, it is certainly not a negative and any extra realism is an advantage when targeting wary or line shy fish.
Trout want to eat a live worm, while they spit out plastic worms.
When trout grab your bait, they usually decide within a second whether to swallow or spit it out. In very clear water, it is actually possible to watch a trout reject and spit out a lure or fly not to their liking.
When the bait is real, they are much more likely to keep it in their mouth for longer which gives more time to set the hook. One downside to fishing a real worm is the increased chance of hooking them in the guts. This is because trout enjoy the taste of worms so much that they swallow them, hook and all whole.
If you are wanting to release your catch, I advise against using real worms for bait.
Less polluting
One of my biggest complaints regarding plastic fishing lures is the mess they create. Take a swim at any popular swimming beach and chances are the bottom will be littered with various soft plastic lures.
Some might eventually biodegrade, but the majority just seem to stick around. During periods of low water levels, I have picked up handfuls of lost and discarded soft plastic lures.
Advantages of plastic worms
Stay on the hook better
Artificial worms stay on the hook a lot better, the main advantage this brings is that they are unlikely to fly off during a powerful long distance cast.
It is also possible to fish artificial soft plastic worms with plenty of action and there is a fairly low chance they will fall off. (Although, I do find quite a few lost plastic baits, so a few still do come off).
So artificial worms allow more time to be spent fishing, rather than rebaiting hooks.
Longer lasting and more durable
Plastic worms are significantly more durable. They are basically plastic so tend to last forever, I have fished the same plastic worm multiple days in a row and they show relatively little damage.
It is even possible to catch multiple trout on the same worm. Real worms on the other hand, usually come off or get destroyed after a few casts.
Comes in a wide selection of colors and sizes.
Real worms come in various shades of browns and reds. While artificial worms come in all of the colors of the rainbow.
A brightly colored worm can be useful to fish in dirty water where there is less visibility. They might even be able to trigger a strike out of aggression if they appear to be obnoxious enough.
It is important to remember when trout fishing and trout often grab a lure out of anger, rather than hunger. So a weird color worm is more likely to trigger a territorial response from fish.
Artificial worms can often be fished where baits are restricted
In some areas, the trout fishing regulations ban the use of bait which obviously will prohibit the use of real worms as bait..
Artificial worms, especially unscented plastic ones are considered to be a lure, not a bait. This difference allows them to be fished in spinning only waters.
Plastic worms can represent more than just a worm
Plastic worm lures, do not always look the most worm like but just represent a generic looking skinny creature.
This means trout can mistake them for all sorts of prey species. If a trout was chasing minnows, it might easily mistake a plastic worm for a minnow, if they are eating leaches. It might be taken as a leach.
The generic appearance of many plastic worms is a strength because they can grab the attention of trout feeding on a wide variety of prey species. Half the time I fish plastic worms, I am convinced the trout mistook them for something else.