Here are the core differences between the 2 games:

1. Exclusive Legendaries – Their legendary Pokémon are Zacian (Sword) and Zamazenta (Shield), which look to be based on wolves as previously speculated.

2. Exclusive Pokemon –

Pokémon exclusive to Sword

Deino
Zweilous
Hydreigon
Jangmo-o
Hakamo-o
Kommo-o
Sirfetch’d
Pokémon exclusive to Shield

Larvitar
Pupitar
Tyranitar
Goomy
Sliggoo
Goodra
Galarian Ponyta
Galarian Rapidash

3. Exclusive Gym Opponents – Pokémon Sword and Shield will have gyms that are exclusive to each version. Pokémon Sword will challenge a leader named Bea (fighting type gym leader) and Gordie (a rock type gym leader). Pokémon Shield will challenge Allister (a ghost type gym leader) and Melanie (an ice type gym leader)