Part one of an occasional series about SmoothWall Express (SWE).
On my machine, the saving in memory is not massive. As observed in the thread, openntpd needs to run two processes (with one running as root), while ntpd can use Linux capabilities. From a cursory look at the Debian bug tracker, ntp has more bugs, but they don't seem as important as openntpd's. I suspect ntp gets more testing. The latest portable release of openntpd was in May 2006.
If I ruled the world, I'd choose ntpd over openntpd.