For most riders, in most cases, a longboard is easier to ride than a Penny particularly if you have never stepped on a skateboard, longboard or Penny Board previously. The main reason behind that is Penny Boards have a very small standing platform, and super narrow shape, that makes balancing harder — going too far forward or too far back means losing control or tipping over forward.
That problem is amplified by the fact the small standing platform on Penny Boards means your feet will be closer together (more narrow / tight stance) — which also reduces control and makes it harder to balance.
On a small 22 inch board that wasted space that isn't super useful and doesn't expand the standing platform on the nose of a Penny Board is one reason a lot of people don't like Pennys in general. Shorter stances also make it harder to recover if something goes wrong and generally results in more wipe outs.
On the other hand a longboard has a much wider effective standing platform resulting in more stability and comfort for cruising (and learning) than the dramatically smaller Penny — which is just a bit bigger than the length of your foot.
More standing platform makes it easier to Balance on a Longboard
Longboards are going to give you a much longer and wider standing platform for balance while also being more forgiving for pushing than a skinny little piece of plastic on wheels. More room for your feet means a more comfy ride — compare that to the incredibly small amount of space the Penny Board allows (which is even worse if you are a taller or heavier rider).
Bigger Better Wheels on Longboards = Less Pushing and easier Cruising.
The bigger wheel size used on Longboards is also going to roll over rocks, cracks and bumps way easier while a Penny Board with small wheels will hang up on cracks and rocks — that usually will unexpectedly buck you forward (possibly onto your face) and means riding a Penny Board requires way more attention, just to make sure you avoid those rocks and cracks.
Penny Board wheels are small (which slows them down and requires more effort pushing) but they are also made out of less expensive, lower performance, urethane that slows them down further making cruising long distances harder on a Penny. It's way easier to push fewer times and coast / ride further but most importantly being able to coast & ride with less pushing means you will improve faster as a beginner on a Longboard since more time is spent riding and less time is spent pushing.
Having larger good quality longboard wheels made with performance urethane and supportive cores is going to help you ride around town, coast further, learn faster and handle whatever features the street has to throw at you.
Easier Slides on a Longboard let you control your speed
That same urethane also makes a lot of tricks / slides easier on Longboards than they are on a Penny Board. The poor quality Penny Board urethane doesn't slide well and the narrow shape (with no concave) means it's not easy to slide a Penny Board and even it was the instability of the Penny setup means they are difficult to control when riding down hills.
Longboard trucks make turning easier
Even the trucks will help create more stability for your ride on a longboard — because of their wider size and reverse kingpin design longboard trucks also make turning easier than a traditional kingpin skateboard truck that you'll find on a Penny. Plus the Penny trucks are incredibly narrow to match the already super narrow width of the board making it more unstable and sketchy to learn on than a longboard.
Penny Boards are cheap — but they are not easy to learn on and even experienced riders need to pay more attention to make sure they avoid cracks, rocks, and hills in general. The only place a Penny Board is really better than a Longboard is if you need to fit it in your backpack, or skate through a crowded airport — in those instances the small size makes the Penny Board an awesome option.