One of the reasons they sell 6" dados is for underpowered saws. It takes more torque to run an 8" set. I ran a 6" set when I only had a 1hp direct drive Sears saw and I'm pretty sure I would have had problems with an 8" set. I also have an Ibox and I'm not aware of needing an 8" set for it? You would rarely need to extend more than 3/4" and a 6" set will do that.
You want a set that is guaranteed to leave a flat bottom and not leave "bat ears" at the corners. There are several different brands that are good such as Freud, CMT, or Amana (and others that are maybe less common). Just make sure they say flat bottomed. I know in 8" you can have a choice in 12 teeth or 24 teeth but I can't remember what the options are in 6". More teeth mean a smoother cut but more teeth require more power and cut a little slower.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.