Conceptual question - matrix items and transaction item options

Let's pretend I am selling widgets which come in different sizes and colors.

I am trying to figure out what the pros/cons are of setting up the item options in the following two methods - like what is the difference exactly?

Method A: I create a matrix item with mutiple "child" matrix items for all of the different size and color combinations. The widget "Color" and "Size" options dropdowns come from custom lists I created, and these options are sourced from the "Color" and "Size" custom item fields for each matrix item. (This is how a couple of products were set up by my client, but the con seems to be the number of matrix items.)