It is not easy to obtain a definitive answer on the wet food/dry food issue, primarily because many of the studies are sponsored in part by manufacturers of one or the other type of food. Veterinarians and other animal experts have also weighed in on the subject for decades, with a general consensus that there is no consensus. A diet of dry food appears to have an edge over a wet diet, but a mixture of the two is also a popular recommendation. To muddy the pet food waters a bit more, there is now a category of "moist" foods which straddle the line between the two types.
The main argument for a mainly dry food diet seems to center around the animal's dental health. The action of eating dry food helps to strengthen a pet's jaw muscles, along with satisfying a cat or dog's natural chewing instinct. The dry kibble is believed to remove some plaque and tartar from a pet's teeth through mechanical scraping, which is further aided by the animal drinking water after eating. Wet food alone does not provide this abrasive action. But, there are of course, additional measures you can take to address dental hygiene, and kibble alone may not prevent all potential dental problems your pet may face.
Some argue that wet food healthier because it is closer to a diet found in an animal's natural environment. A cat's natural diet, for example, would be high in protein and low in carbohydrates. Wet food for cats is usually high in proteins, fats and moisture, while dry cat food is often higher in corn-based carbohydrates, something a cat would not normally find in nature. Because dry kibble lacks much moisture, it is recommended that dry food diets be supplemented with sufficient amounts of water. With respect to protein content, dry food actually contains a fair amount of protein, but wet food proponents say it is not as natural as the proteins found in their preferred food. The additional fats are also believed to encourage healthy coat growth.
You'll find veterinarians recommending a variety of diets. Some recommend a primarily dry diet, with wet food used as an occasional treat or supplement. Kibble is considered more economical, since it does not spoil as quickly and is often sold in bulk. Many pets tend to prefer the aroma and texture of wet to dry food, which is why it may be difficult for owners to switch to an all-dry feeding program. Mixing some dry kibbles with the wet meal may be enough to prevent dental problems and give the animal an opportunity to wean itself off the wet pet food gradually. Wet food can also spoil quickly after opening, so it should never be left in the animal's bowl for excessively long periods of time. Any extra should be refrigerated or thrown away.