u can't compare the games tbh. Legends Arceus is a totally different game, therefore u wouldn't play the usual Pokemon experience, which might be a good or a bad thing, it just depends on u. Tbh PLA is not out yet, so we can't truly know how the game is gonna be. Imo it's the game that many pokemon fan actually wanted, especially if it's going to have a good lore. As I was saying, it depends on what are u looking for. Despite being a main series, PLA is more of a Spin-off tbh, while Sword is a real main series game. The two games are too different, but it depends on ur priorities: do u want a very different game, with few mons and probably a good story? PLA could be that. If u like a normal pokemon game, with many areas to explore, with much content (if u get DLC) and if u re interested in competitive, just go with sword.