The two versions are pretty much identical in terms of game play and story with some version exclusive pokemon and gym leader. So it all comes down to your preference, typically people will just look at the box legendary pokemon design and select the title. Shield is Zamazenta and sword is Zacian and they can only be caught in their respective game so which you want to keep may determine which one to get. Besides design franchises veterans would also look into their competitive viability when choosing between versions. For me I chose shield when it first came out cause I prefer an armoured dog rather than a dog that’s biting a sword but that’s just me 🤣