Starting out, it's worth knowing that ward is probably easier for newcomers since you'll have access to those military bases and better equipment from the start. The chemical plant base and stc malachite are super handy fast travel points, and you'll get decent missions like law and order that aren't too complicated. Tip - if you're into the military approach, grab that psz-12v bulat armor early on since it helps a ton with surviving firefights especially against mutants.

Now if you're interested in the deeper zone stuff and previous stalker stories, spark might be more your style. You'll team up with some interesting characters like scar and strider which longtime fans will recognize, but keep in mind you'll have more hostile encounters and won't have access to those convenient bases I mentioned. The missions are a bit longer and more complex, but they dig deeper into the zone's mysteries.

Something interesting - both paths can actually lead to any ending you want since you can switch sides later. You could start with spark and still end up finishing with ward if you play your cards right during those final missions. Just remember that whichever you pick, the other faction won't be too happy to see you around their territory for a while.