GnuWin32 or Cygwin - from a programmers point of view
It is absolutely no contest
GnuWin32 does not include Bash
This is a critical part of the Cygwin distribution. Switching from cmd.exe to Bash was a huge boon to my productivity. cmd.exe is cripped in comparison to Bash.
GnuWin32 is old
Many program go years without being updated.
tar2003coreutils2005zlib2005make2006wget2008
I would not use GnuWin32 unless I had no other choice.