Penny Boards and Longboards both share four wheels and a standing platform — but other than that there really aren't many similarities. A longboard is designed to give you the best cruising experience by making use of a large foot platform (easier to stand on) where as a Penny has been designed as the very smallest possible platform with the primary goal to make the purchase price of a Penny Board as cheap as possible.
The focus on inexpensive price at the cost of ease of use and performance impacts not just the Penny Board shape but also the selection / choice of the components that makes up the complete board.
Longboard Wheels vs Penny Board Wheels
Starting with the wheels, longboard wheels are soft and larger than traditional skateboard wheels. The development of urethane formulas used in Longboard Wheels have been a huge focus for years across the longboard industry — with specifically selected options to make sliding (to control your speed) easier, roll speed higher, or grip better.
Longboard wheel formula development has been driven by the professional downhill longboard racing circuit as well as a large number of free skate / free ride events which continue to see riders push the boundaries of their wheels — and wheel developers respond with improved wheel shapes, cores and urethane to better address those needs.
Penny Board wheels are again set smaller, not because Penny Boards can't fit a larger wheel (in most cases they can — and generally would ride better) but rather because a smaller wheel uses less urethane which is less expensive. Again this leads to to a cheaper price overall for Penny Boards at the cost of performance which primarily impacts new riders / beginners who are just learning.
Penny Board wheels also make use of less expensive, poorly performing, urethane formulas rather than the performance oriented formulas selected by premium longboard wheel brands. Generally that leads to Penny Boards rolling slower, requiring more pushing, and being more difficult to slide on hills — meaning it's harder to control your speed and way more dangerous to carve even moderate hills on a Penny Board.
Penny Board Bearings vs Longboard Bearings
Looking closer at what keeps you rolling, even the bearings on Penny Board trucks are once again cheaply made, incredibly slow, and break down easily. While longboard bearings are made specifically made for skating and some with upgraded built in components like speed rings and spacers designed to roll even smoother, make it easier to swap out other wheels, and clean at ease.
There ARE also cheap longboard bearings that have the same problems mentioned above but MOST Longboards come with a much higher quality bearing than the standard Penny Board.
Penny Board Trucks vs Longboard Trucks
When it comes to the trucks, reverse kingpin longboard trucks are much more stable at both high speed (downhill) and lower speeds (pushing around town) when compared to Penny Board trucks. That makes it easier to cruise and easier to learn to balance as a beginner.
Penny Board trucks were specifically made more narrow — leading to less stability — not because larger trucks would not fit (they would), but rather because smaller more narrow trucks are less expensive.
A wider Longboard truck with bigger bushings (made of quality urethane) and efficiently designed turning geometry is purpose made to help you learn faster, ride faster, and stay in control. There is no better way to maintain the stoke between you, and your board than to stay safe and in control no matter what hill / terrain you happen to be riding.
Longboard Shapes vs Penny Board Shape
Longboards have so many different shapes to fit your style for cruising — from drop throughs to pintails, kick tails, drop decks, mini cruiser's...the possibilities really are endless. The reason there are more longboard shapes than Penny Board shapes is that the molds that are required to form the plastic used to build Penny Boards are incredibly expensive — that means small improvements that could be made to help Penny Boards perform better are almost never addressed.
Longboards on the other hand are pressed in short runs and do NOT use industrial injection molding — that means improvements and changes are made frequently, problems are found and fixed, and your longboard is more likely to perform the way you want it to straight out of the box.
Buying a Longboard will not only last you in the long run and be more fun, but supporting core longboarding brands will give back to the sport and community as a whole.