Stsadm Tool: Microsoft Office SharePoint Server 2007 includes the Stsadm tool for command-line administration of Office SharePoint Server 2007 servers and sites.
Stsadm is located at the following path on the drive where SharePoint Products and Technologies is installed: %COMMONPROGRAMFILES%\microsoft shared\web server extensions\12\bin. You must be an administrator on the local computer to use Stsadm.
Windows PowerShell: Windows PowerShell command-line interface is a new command-line tool and supporting scripting language from Microsoft that complements Cmd.exe in the Windows administration context. In the SharePoint administration context, Windows PowerShell supersedes the Stsadm.exe administration tool.
Return Parameter :
Windows PowerShell:Unlike most command-line tools, which accept and return text, Windows PowerShell is built on the Microsoft .NET Framework and accepts and returns .NET Framework objects.
Access to File System :
Stsadm : It does not allow you to access file system,registry ans so on..
Windows PowerShell :Like many shells, Windows PowerShell gives you access to the file system on the computer. In addition, Windows PowerShell providers enable you to access other data stores, such as the registry and the digital signature certificate stores etc..