If you are making a quilt that will become a family heirloom, it is worth it to buy the best quality fabric you can afford. The difference is the thread count. The more threads per inch, the better the fabric. Sometimes you'll see the better brands on fabric at Joann's, but it is printed on lower quality cotton.
I will use Walmart or Joann's on wall hangings or other sewing projects, but for the quilt you described, you should use the best you can.