The answer isn't a simple yes or no - it depends on the cat and where you live. Most veterinarians, including all of us at The Cat Doctor & Friends, tend to favor a safer indoors only lifestyle.
That said, not every cat can be happy as an indoors only cat. For owners who want an indoors only cat, I encourage adopting a kitten who has never been outdoors. As I like to say, "What a cat doesn't know, he doesn't miss." Cats who have spent significant time outdoors can feel imprisoned indoors although some do adapt well, especially timid or anxious cats. Many outdoors only cats are grateful to come indoors-no more looking over their shoulder for unknown dangers.
The advantages of an indoor only lifestyle are obvious - no coyotes, no cars, no dog bites, no exposure to unknown poisons, no exposure to fleas or ticks or other parasites and no exposure to most infectious diseases. Indoor only cats tend to live longer and it is easier to monitor a cat's health when he or she lives indoors, since an indoor cat must use a litter box and can only eat and drink what you offer him. So if your indoors only cat stops using the litter box, or starts drinking a lot of water, you will know right away. Hint - calling your veterinarian should be your next move.
Outdoor cats tend to use the outdoors instead of a litter box which is one of the advantages, at least for the owner, of an outdoors lifestyle at least as far as convenience goes. While I can't say I love litter box cleaning duty, newer litters have eliminated litter box odor as a major problem if you scoop conscientiously and wash litter pans weekly.
The other advantage of an outdoor lifestyle is the avoidance of obesity, which is a major problem for many indoors only cats. Outdoor cats do tend to burn more calories, especially if they hunt. The bad news about cats who hunt are their exposure to tapeworm if they hunt and eat rats or mice and the significant decrease in songbird populations wherever there are many outdoor cats.
In order to prevent an overweight indoor cat, try environmental enrichment and avoiding the common practice of free feeding (a never empty bowl of dry food). Environmental enrichment involves window perches, "cat trees", toys and playing with your cat. Some cats will fetch, others will jump at a string waved in their face and some prefer an empty paper bag or box to explore. A newer idea is the "food ball" which is a ball filled with treats that the cat must manipulate in order to get a treat. For more ideas, go to www.catvets.com/healthtopics/wellness.
Because indoor cats depend on us for entertainment, I believe that we develop deeper bonds with our indoor cats. They get to know us and our habits and we start to understand their little quirks, likes and dislikes. It's a lot like a good marriage. Having an outdoors cat is more like dating….non - exclusively.
One last thought - it seems to be a very common myth among cat owners that if a cat never goes outdoors, he or she won't need to go to the veterinarian for annual check ups. While it's true that an indoor cat will probably have fewer health problems, he or she can still have dental issues or allergies, diabetes, kidney, liver or heart disease. They'll need fewer vaccines then outdoor cats do, though.
In conclusion, I vote for indoors over outdoors for many reasons. If your cat absolutely must go outdoors, please keep them inside from early dusk until well after dawn, to decrease the chance of a coyote encounter. Please consider an outdoor enclosure as a compromise - good examples can be seen at www.habitathaven.com.