Fast answer
Choose Dewgrub for stronger overall Water value. Choose Frostnip if your main problem is lane pace and you want more slow control than raw carry value.
What Dewgrub does better
- stronger general-purpose Water damage
- better long-fight value
- easier to justify as a major evolution target
What Frostnip does better
- more slowdown utility
- safer handling of messy waves
- better fit on teams that already have enough damage
Verdict
Dewgrub is the better broad Water investment. Frostnip is the better “I need more control right now” answer.