runas is already installed on many systems (maybe by default)
psexec is shipped with the awesome PsTools.
Why would I use
psexec (an external tool) instead of
runas(an included tool) ?
What are the differences between them?
Of course psexec is no longer an external tool - SysInternals is now part of Microsoft (since 2006, I believe)
From Mark Russinovitch:
Have a good look through the psexec page here.
One of the most important features of psexec (at least for me):