Buy the sheets. They machine quilt easily, wash and wear really well and are an inexpensive way to back your quilt saving you money to buy fabric for tops. Unless it is a really special quilt, I use sheets for backing all the time and have had excellent results. I even bring the backing to the front and top stitch it down for binding on many of my everyday quilts (don't tell the quilt police!). It's your quilt, try it out!