I've owned two Sigma flashes and found them to work very well. They also took a lot of abuse/drops without any apparent damage. I still have them. I switched and stayed with Nikon when I ran into compatibility problems as I upgraded bodies, but Sigma seems to be doing a better job these days maintaining compatibility (with the exception of the super model and the D7000).
The SB 600 is a great suggestion, but they seem difficult to find in the Yonguo price range.
Personally, I think I'd opt for a Sigma or Nissin over the SB-400. They're more powerful and flexible for directing the flash where you want it.