This turned out to be a very interesting question. There are lots of links out there on this topic but they are ambiguous in describing the difference/relationship between the two. Best description I've found of the hard drive configuration for a Windows OS is this one: http://www.ntfs.com/hard-disk-basics.htm.
It appears that MBR and Partition Table are in the same sector on a drive. The MBR is "smaller" in that it is the very first thing on the drive, that then uses the partition table to continue the boot process to a specific OS. The two command options effectively fix different links in the boot chain:
/fixmbr replaces the information and small executable that reads the partition table to find where the OS may be located. So this exists on any drive that has been formatted and effectively exists to read the next little bit on the hard drive that tells where the/an OS is supposed to be located. In essence, this is not necessarily a Windows-specific item.
/fixboot replaces the next part - the entry in the partition table that points to where the actual loadable executable is located for the OS. So this is fixing the next link in the chain of the boot process. This command does create a windows-specific result in that it reminds the hard drive where to find Windows in particular.