It all depends on the type of quilts you like to make.
If you like to make quilts that use a lot of smallish shapes, like half square triangles, then you'd probably want to get some half square triangle dies.
I tend to use my 2" square, 3.5" square and 5" square dies more than any of the others that I have because I like to cut scrap pieces that are smaller than a 10" square into sizes and shapes that are easy to store in my tiny space and those 3 sizes are ones that I can use in the kinds of quilts that I like to make. I have one of the cubes, the 9" block. That one has a number of dies (including the 5" one) that I had been looking at as separate purchases, so it was worth it to me to get that one since I knew I would use most of those dies.
If you go on the website, you can see which dies are included in the cubes and decide if that would be a reasonable way to go.
Rob