From Apple: HFS Plus via Erase vs. Initialize

A new Apple TIL file documents the difference between using the Finder's Erase Disk command and Drive Setup's Initialize command as a means to convert a volume to HFS Plus format: "Erase disk will only initialize the Partition selected. Drive Setup Initialize will reset the Boot Block and Partition Map as well as the Partition(s) selected. So in fact Drive setup performs other routines that Erase disk does not."