Jju and vicky did good answer.
"anniya" is like when your teasing someone and they say "anniya..."
=>anniya is very similar with ani. both are informal. but 'ani' can't use end of sentence.
ex 1) 이건 내 가방이 아니야. (O)
이건 내 가방이 아니.(X)
A: 이거 네 거야?
And if situations are like below, 'anniya' is more suitable.
A: Hey, Did you break up with boyfriend?
B: I couldn't hear you. What did you say?
C: I really thank you for your help.
D: (you're welcome.) anniya.^_^