External Flash Nikon SB 900 vs Nissin Di 622

Hello masters.

I recently bought a Nikon D 90 camera and the lens 18-105 AF VR. I am planing to buy a new external flash unit, I have options that Nissin Di 622 or Nikon SB 900 or SB 600. which one should i prefer for better results??? and which one has a better performance?? however I want to meet it my budget aswell..

Pls advice

Regards
shamdesigns/vidya creative media