if i plan to use a solid color on the back of a twin size or smaller quilt, i often use sheets. it saves tons of money and i know they hold up nicely.
for larger quilts, or those for which i want to use a printed back, i usually go with regular fabric because both options cost about the same. (our local thrift shops don't offer many bargains. if they did, and i found pretty sheets there that were in good shape at a decent price, i'd go with the sheets).
if i ever had the guts to enter anything i'd made into a competition, i'd go with quilting fab for the back. the quilt police have fits about sheets. :roll: