3.13. Which should I use? MKS or Cygwin?

jtreg supports both, equally. However, the tests in the JDK regression test suite assume that MKS is available. So, when you are writing tests for that test suite, you should make sure that your test at least works with MKS. If you prefer to use Cygwin, and can make your test run with both, so much the better.

Note: as of JDK 8, the tests assume the use of Cygwin by default.