Penny Boards are best used for pushing short distances in crowded locations (with no hills) where you might need to stop to pick the board up often. Having the smallest board possible when there isn't much room (in an airport, or a crowded tourist location) is a big advantage
BUT that advantage comes at the cost of comfort, control (and safety), and performance on longer pushes, riding down hills, or when doing tricks.
What should Penny Boards NOT be used for?
Penny Boards are NOT good for tricks, not good for cruising and carving down hills, uncomfortable to push long distances and dangerous at higher speeds. In all of those cases you would be better off (and safer!) getting a longboard or a skateboard instead of a Penny Board.
Penny Boards can be difficult to control and Dangerous at higher Speeds
The narrow width of Penny Boards, and short wheelbase / standing platform, means that it's harder to recover from a mistake and generally leads to more wipe outs (and injuries).
Should I get Longboard or a Penny Board?
As mentioned above Penny Boards are incredibly specific in what they are best used for — if you get a Penny that means most of the time you will be better off leaving your Penny Board at home and using a different board (or even a bike).
If you are cruising, carving and commuting Longboards are a great choice for comfortable riding in full control. That control also lets you learn faster as a beginner than you would on a Penny Board — and allows you to safely control your speed on hills.
Do Longboards last longer than Penny Boards?
While Penny Boards use cheap plastic that is both worse for the environment and weaker, Longboards are made of a full range of performance materials. Some of them use Carbon Fiber reinforcement to improve performance for tricks and stay unbelievably strong.
The Apex 40 Longboard family is meant to do a little bit of everything — but performs incredibly well while doing tricks and will last MUCH longer for riders that want to do tricks (and are hard on their boards) than a cheap Penny Board made of plastic.
Are Longboards or Penny Boards Better for me?
For pushing longer distances a Longboard is and even better choice when compared to a Penny Board. The wider platform and ability to load bigger wheels makes longer distance pushing, cruising and commuting more fun and more comfortable. It also opens up the possibility of taking longer trips using nothing but your longboard.
Road trips and world travels where you may end up riding incredibly varied terrain cause major problems for a Penny Board since many times you may find yourself wanting to ride SERIOUSLY steep hills. If traveling is a huge part of your life there are huge selection of longboards that will fit in the overhead compartment and give you much more versatility to handle whatever roads you find while you are exploring.
Should I get Skateboard or a Penny Board?
Skateboards are different than longboards in that most longboards are primarily designed to make it's tightest turns (and slides) with your feet (stance) either over top of the trucks or BETWEEN the trucks in the center of the board. A skateboard on the other hand is designed to make it's tightest turns with one of your feet on the kicktial — by lifting the nose of the board up.
While there are a lot of Longboards designed specifically for tricks (see the Apex 40 DiamondDrop above) most Skateboards prioritize performance on ramps in Skateparks and in cities above cruising, carving and controlling your speed on hills.
Penny Boards on the other hand prioritize PRICE (ie. being cheap) and being small which limits what they can be used for and makes them VERY bad choices for tricks around town, in cities or riding in Skate Parks. Skateboards on the other hand are the perfect board choice.
The fact that Penny Boards are so narrow and uncomfortable also makes them a bad choice for city riding / cruising since pushing long distances is both uncomfortable and less controllable — there is also no Kicktail to allow you to ollie up and down curbs, or over manholes / obstacles that show up pretty often when skating in urban areas.
All in all if the question is: skateboard vs longboard vs penny board it's pretty rare that getting a Penny Board is your best option.