Prefer Janie to Janey... and Jenny's boring... and has the added drawback of also being the word for a female donkey. On the other hand, if the character was born in the 70s or early 80s, it would fit the era.
It depends on the character. Janey is more feisty and opininated. She is a leader, take charge kind of person. Jenny is sweet and likeable but quiet or shy.