Spray basting for me, but it helps a *lot* to know a few tips before you start. Such as, mark the centers of the sides of your backing, batting and top; really helps when lining them up. After lining up batting on top of backing, fold back the batting and spray baste just one-half at a time.
If overspray is a concern, tape down the largest flat sheet you have first, to catch overspray. Afterwards toss this sheet in the laundry.