my favorite stabilizer is a knit iron-on...it still allows the knits to drape well or "flow" so they're softer than the woven stabilizers when finished. The muslin being used by some posters is a stabilizer...just a sew-in one and if that gives you the feel you like, go for it. The sprays will probably wash out after the first laundry, so determine if that is a problem for use of the quilt.
I suggest that you ask the customer how much the quilt will be used. If it's only to be hung and for show, you may not need a stabilizer at all - except maybe the spray style to keep it in place while you sew it. If it will be used a lot, I encourage you to use some kind of stabilizer because even after sewing the knit fabric can and will stretch when being used and will wear away faster that way.