What factors go into choosing between 3-sided and 4-sided capping for composite deck boards?
What factors go into choosing between 3-sided and 4-sided capping for composite deck boards?

TL;DR
Cost and how long do you want it to last.

I'm not a decking expert but composite decks with 20% wood fiber content are still prone to rot, albeit at a slowed pace. What you end up with is something like this: https://www.youtube.com/watch?v=ahiI6WTumB8
By having all sides capped you reduce the surface area for environmental variables to affect your boards.
If you switch to 100% polymer then you simply get to enjoy the longevity of the plastic and not worry about the wood filler. These too will degrade over time as nothing is safe from the sun. Maybe a stainless steel deck will provide the greatest longevity but you'd have to consider the cost.
One of the most important things to consider with any deck is joist spacing. If your joists are closer together then your deck will feel less spongey as the years go on. Also, if the span calls for a 2x8 joist then consider using a 2x10 or 2x12 if you really want it to feel sturdy.
Keep your deck swept, washed, relatively dry, and don't let microbes deteriorate your deck over time.