A 5’ cube covers 4 squares on a grid. A 15’ cone covers approximately 6 squares on a grid. So that’s potentially stronger, though with poison damage being more frequently resisted than acid that might be a reasonable tradeoff.The new acid splash does 1d6 damage in a 5' cube. So there's your metric. Reduce poison spray to 1d6 damage in a 15' cone.
You're also looking at melee vs 60".A 5’ cube covers 4 squares on a grid. A 15’ cone covers approximately 6 squares on a grid. So that’s potentially stronger, though with poison damage being more frequently resisted than acid that might be a reasonable tradeoff.
Poison Puff?I guess should maybe be called poison squirt/spurt than poison spray, mm?
A 5-foot cube is one square. A 10-foot cube is four squares.A 5’ cube covers 4 squares on a grid. A 15’ cone covers approximately 6 squares on a grid. So that’s potentially stronger, though with poison damage being more frequently resisted than acid that might be a reasonable tradeoff.
Nope, on a grid AoEs always originate from corners.A 5-foot cube is one square. A 10-foot cube is four squares.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.