What is better: sucking on thumb or pacifier?

Pacifier. If given a choice, sucking on a pacifier is better than on a thumb. A parent can make the pacifiers disappear when the child is old enough, but it's hard to make the thumbs "disappear". Also, sucking on a pacifier won't cause the calluses and cracked skin on the thumbs.
My choice, pacifier. Pacifiers can be a transitional object, replaced by other self soothing objects like teddy bears.Some develp a tight bond with it & will require creative methods to reduce us.A clip/ribbon keeps it close & avoids using the mouth as a pocket.A ziploc bag allows visability without immediate access.Sudden loss allows the thumb(that will not go away) to be a substitute & leads to mistrust of caregiver.