Choose your character!
That’s part of it yes but including either of these structures into your story can be a useful tool for organizing plots. Order and familiarity help ground the audience’s expectations while giving the writer the means to subvert, confirm, or reject those expectations. You can’t pull the rug out from someone if you don’t lay out a rug. Nor can you build a house in a void. But when organizing your plot you’ll have to think what makes the most sense for your story.
In short, stories of simpler design generally benefit from the rigidness of the Freytag’s Pyramid structure while more complex stories as well as episodic tales can be fleshed out with longer story arcs.
The basic inciting incident, rising action, climax, resolution plot is so ingrained that people can let it just pass by. An educational story where you’re just trying to get across a good moral or some basic history lesson are best left digestible. If the focus is on likable characters and escapism and spectacle such as a Star Wars, then it’s best not to divide the audience’s attention with a grandiose exploration of a brainy concept. However, the same simplicity can create such a lull in expectations that a tiny twist here or there can be magnified in importance. There’s good and evil, black and white, until that one scene in Empire.
A longer story can take the time to flesh out characters and philosophize on themes. If you want to have a larger scope the central focus could be on a monarchical throne and the different rulers sitting upon it over generations. How does each king or queen contend with the trials of governorship? To get specific, let’s have the progenitor of the dynasty kick a can down the road—say he deals with a food shortage by sending his peoples to plunder the nearby coastal lands. He does this knowing many will die but considers it a mode of population control and a chance to eliminate challengers to his throne. The negative political fallout and lack of skilled soldiers will be the next ruler’s problem. Then when that ruler kicks the can down the road into a more precarious situation we have a dangerous cycle. The people come and go, the capitol may change locations, ships gone to sea never to be seen again become myth, but the concept and consequences of that shared character flaw remain for the entire story.
If you’ve got 300,000 words to work with then that’s a long time to just build tension. You can’t simply multiply every segment of the pyramid and have an introduction plodding well into the tens of thousands of words and a pivotal climax which needs to be stretched to meet quota. Likewise an episodic story (I’ll offer up the medium of manga) can’t run an issue every week or month for years building to a single event. There have to be intermittent goals for the characters to strive for to maintain pacing.
Manga is interesting with its constant use of arcs (which often get their own names) because they contain strong blends of story arc based plotting and traditional plot structures. The overarching goal of a widowed warrior attempting to raise his son to be a better person than himself – perfect for a long-form decades spanning storyline – can be broken up into smaller arcs where the father is met with an obstacle which challenges his moral integrity. Each obstacle can then follow Freytag’s Pyramid to the appropriate degree. To keep them from getting stale have a high stakes external conflict arc be followed by a lower stakes introspective arc. You may not be able to stretch the pyramid out but the alternating roller-coaster action between chapters can be used for arcs just fine.
For smaller series, a duology or trilogy, a nice, well-thought-out pyramid would work wonders for each installment. Shorten up the introduction on the sequels to not repeat exposition and vary the pacing slightly to keep the plot less predictable. Add in a few overarching elements like a flourishing love, a yet to be challenged big bad, predictions/fortunes, character transformations, etc. and you can keep an audience entertained the entire time.
To break it down for different types of writers I’d say Freytag’s Pyramid (and other templated plot structures) is for architects and story arcs are for exploratory writers. They aren’t exclusive but if you’re going to plan out the events in rich detail, they’ll likely fit puzzlelike into a common story structure. If you’re writing and learning alongside the characters about what they really want at their core then that’s going to be an overarching theme in the tale.
But try both. I implore you to give them a shot and then mix them up in different ratios till you’ve found what works best for you and the stories you want to write. Then when you wish to get out of your comfort zone switch-up the recipe.
When you’re not really done then there comes the edit where you have to see if the pyramid is too obvious or if the arc is too obscure, or if whatever Frankenstein’s monster you’ve created has to be looked at sideways to be seen as a story. Perhaps another time. Best of luck with what you choose.