Hobbs 80/20 and Quilter's Dream cotton are both excellent for machine quilting as well as hand quilting.
You do not have to match batting to fabric content. Different battings create different effects, so choice depends largely on what you want the end result to look like. You can use wool batting with cotton fabrics, silk batting with cotton fabrics, cotton batting with cotton/polyester/silk/wool fabrics.
As someone mentioned, some battings hold together with quilting lines up to 10 inches apart (these are the battings that are needlepunched through scrim); others need to be quilted 2 inches apart (usually battings that are not needlepunched at all). So, how closely the batting will be quilted can determine which one you buy.
Loft is another consideration. For machine quilting, it is much easier to quilt a low loft batting than a high loft batting. Quilter's Dream cotton is low loft. Hobbs 80/20 is a modest medium loft (between low and medium) and also easy to machine quilt.
Polyester battings are more slippery than cotton and therefore can be more difficult to machine quilt.
Shrinkage is another consideration. Most high-quality brands limit shrinkage to 3% or less. This is true of Hobbs and Quilter's Dream. Someone who does not want any shrinkage at all might go with a polyester batting because polyester doesn't shrink. If used with pre-shrunk fabric, the quilting lines will show very strongly with a modern "harsh" edge. Most of us want at least slight shrinkage in our quilts, as it gives a softer look. Antique quilts, which are thin and soft, typically used 100% cotton batting that shrank at least 3%.
To be honest, I would just get Hobbs 80/20 for your wife and let her use that for the first time. I'd say that this particular batting pleases the widest range of tastes, and it's easy to use. If, after using the Hobbs, your wife decides she wants a flatter batting, try the Quilter's Dream for the next quilt. But I think she'll be happy with the Hobbs 80/20.