NTP server quality: low delay or low jitter? I got two NTP servers I could use, one is Stratum two NTP, having low jitter and high delay; Second one is Stratum three NTP, having low delay but higher jitter. For the sake of accuracy, which one should I choose? NTP itself take preference on low jitter, not sure if it's a good choice. Any comment?

The delay is compensated for by ntp, provided it is symmetrical, ie similar for both directions, so it is less important than jitter, which stops your local clock from knowing what is the real time.

Usually, ntp is configured to use several servers, and, as you observed, you can trust it to chose the best. 3 servers would be better, just in case one is claiming it's next week.