I like the solid white or unbleached 106" when I can afford it or have it on hand. It is a time saver and I find I have less waste. I have pieced the backs of many quilts using the 44 " and they are just fine. As far a color I tend to use a lot of cream or white in my piles of scrappy quilts so the cream/white back is just fine. With some nice quilting, SID. echo or meandering quilting, the back becomes a pleasant white-on-white quilt.
In the end, do what feel right with that particular quilt.