Technical difference: one is a very basic and simple spacer, the other has studs and is inherently safer. They make adapters that are same/same bolts pattern, so basically a spacer with studs. If you are not changing stud length then a regular spacer under 1/2" is ok, if you are looking to go wider than that, and are set on doing this with some type of spacer, then an adapter would be the way to go.
