fleece because it does not breath---flannel makes wonderful- warm, cozy, soft, fluffy quilts- and the batting makes a difference to- the quilt on my bed has a cotton top, a wool batt and a flannel back- it is the most wonderful fluffy soft warm quilt- but also light weight and breaths so we actually have it on the bed in the summer too....it washes up so wonderfully- just gets better and better.
fleece is fine to use if that is what you want- but does have it's own set of (conditions) when getting ready to quilt it.