Better HP, Special attack and Special Defence
More useful typing competitively
Has access to good status/Special moves
Abilities are really good depending on how you use it..
Doesn’t hit very hard.
Most likely role to play:
Speed is higher than Jellicent
Resistant to some common competitive types
Really good abilities
Not so good HP, Defence or special Defence
Weak to some common types
Slightly more limited Movepool.
Technically bruxish is better in competitive (It’s in NUBL) however Jellicent has proven to be stronger in game.
Edit: You get Bruxish and Frillish on the same route, and at that point of the game you should be able to get it to level 40 when it evolves before the Grand Trial.