Open Pokémon packs or keep them sealed?
It depends on whether you want cards or want to hold product. Here is what each choice means for your money.
Opening packs
Opening a physical pack usually returns less card value than you paid: most packs are commons and uncommons, and the chase cards are rare. You open for the fun and the chance of a big hit, not as an investment.
If you want a specific card, buying that card on its own is almost always cheaper than opening packs until you pull it.
Keeping it sealed
Sealed booster boxes and packs from popular sets have often gone up in value after they went out of print, because every opened pack makes the rest scarcer. That is not guaranteed: heavily reprinted sets can stay flat, and you need dry, safe storage.
Check that sealed product is genuine before you buy it; resealed packs and boxes are a known problem.
A third option
Digital packs give you the opening without the bulk commons: on GrailCart every pull is a real card from a published list with published odds, and you can sell it back instantly or have it shipped home.
See also: vintage Pokémon card values · price guides by set · cheap packs