The only difference is that the 6 x 6 matting Basics B has smaller dies.
Basic A: smallest die is 2.35 in. x 2.35 in. to largest die is 5.85 in. x 5.85 in.
Basic B: smallest die is 2.15 in. x 2.15 in. to largest die is 5.65 in. x 5.65 in.
Each set has 8 dies. Just depends on how large or small you want to make your project.