I would say you should definitely go for the Night Blue Nikki...not only do I love the color, but I feel like this bag would last you longer. The Tye Dye seems like a novelty to me, and once you get sick of it, you'll wish you had gotten the Nikki. ;p Plus, I just like the Nikki shape and style! Good luck with your decision!
I'm buying the Tie Dye but only as a extra fun bag. You gotta have the basics I think before you invest in a bag like this that may not be what you want when you get it. The Nikki is more of a Basic bag. I'm getting the Tie Dye as a "Dare" sort of bag. It's so off beat and odd I just think I had to see it for myself. Hopefully I can pull it off.