In terms of raw “which is more useful”, Chrono hands down.
Chrono supports the party with movespeed buffs, attack speed buffs and heck a CD reduction skill. It’s really great for party play.
Alchemist on the other hand, plays a more “passive” role, by giving potions and weapon enhancements (it’s eh) through Briquetting and Item Awakening.
An alchemist wouldn’t be able to support a party as well as Chrono, cos you can’t like craft potions mid earth tower ya know? I’d say it’s more of a passive character, crafting potions and having fun with magnum opus puzzles. Don’t expect it to rake in millions of silver too, it’s more of a side income.
However, I’m not indicating Alchemist is bad, in fact I really love the class lol. But in terms of party support utility etc., Chrono takes the cake.