IMO, your first consideration should be if you are using a longarm or not. If you're using a longarm, or sending it out to a longarmer, this picture illustrates why you should piece the backing seams horizontally.
Second, if the look of the quilt was more important to me than the economics, I would pick a busy backing fabric that wouldn't show seams. The ideal fabric would be dark with a small and very busy print. Here's a good example:
