RE: Decoys...
When it comes to decoys buy the best you can afford. If weight is not an issue in how you hunt always buy or make good cork decoys for on the water. You canoutdraw with a dozen good cork, properly placed, a person that has out 2-3x's that many.
That being said target the species that is most prevelant to your area. Use that as a starting point and add from there as your budget permits. Here in the Northeast from N.J. to Maine a good choice to build your spread around is Blacks and second is Mallards. Blacks are a wary bird compared to others and all other birds will come to a nice set or Blacks.