Generally, I only measure to be assured that it's of legal length for the state or lake I'm fishing. I'm of the school of thought that length is for legality, weight is for bragging
There's also a big difference in a fish's weight per length, according to the lake/state it comes from, as well as what season it was caught in. Which of the two species of Crappie it is, can also be a factor.
IMHO ... if it matters enough to know "how big it is", then BOTH measurements are needed, plus which species, since "my 17in fish" may weigh more than "your 17in fish" ... and categorizing them as "equal", on length alone, would not be fair judgement.
... cp