Help me decide [Katarina skin choices]

To date, I've played well over 1500 Katarina mid matches in solo queue, and have found great success with her. My only regret? I have yet to buy a skin for her.

Anyone with a legitimate opinion as to why one's better than the other (other than "merc's cheaper" or "warring kingdoms is newest") feel free to comment, I'd love to know why you feel one is superior over another.