With its long skirt, version A should ideally be made from tightly woven fabric that can withstand the weight of the skirt without stretching: rayon crepe (not challis), silk, lawn or cotton voile. With very thin fabrics, you might want to reinforce the waistband by lining or interfacing it, so that it keeps its shape.
Version B gives more leeway in terms of fabric choice! Just be mindful of the drape: the butterfly sleeves of this version will be much prettier in a fluid fabric such as rayon, silk, or double-gauze.
The Magnolia dress can showcase prints particularly well, as long as their scale is small enough so that they are not cut off by the skirt panels! Flowers, dots, plaid, geometric patterns… anything goes!
If you’d like to sew Magnolia out of a solid fabric, we recommend being particularly careful about the quality of the fabric, so the dress keeps its elegant look. If you want to use a cotton, look for heather colors or subtle embroideries that will add depth and texture to the fabric.