got failed exampels of both at work.
decent spacers that bolt to the hub are the better of the two bodges provideing they are made of a sutable material.
i wouldn't trust alloy ones TBH unless i knew exactly what alloy it was made from and looked at the numbers myself. threads have a nasty habbit of stripping, steel is much tougher but heavyer.
the other way if there is plenty of meat in the center of the rim is to drill and tap the rim and bolt the spacer to the rim then bolt though