I often prefer to improve other people' answers instead of writing one of my own (unless mine would be considerably different). To be entirely honest, I do this for petty selfish reasons - to show off that I can, to avoid having to explain the answer myself,
and maybe to gain some reputation.
The typical behaviour I observe then is they either suggest to me I should post a separate answer (which I, being lazy, refuse) or they credit my username with the number of characters I saved. That's why sometimes I only give clues, to make the author come up with the improvements alone, and to avoid complicating things.
What would be a good rule of thumb to tell if an improvement is worth posting as a separate answer?
From the PPCG community point of view, is an unexplained shorter solution preferable to an explained longer one?
Is credit necessary?
Wouldn't it be better to upvote the comment instead of @-mentioning in the answer?