Thanks for all the responses!

1. Leaving the night before isn't really an option. Very short time in Xian.
2. We have spent a week in BJ before so have some familiarity with the city and transportation. (But have only connected once at PEK, and that was on a late evening flight from Kumming, which was really easy.)

We originally planned to take the train from Xian, but then saw that the MU flights are about the same price and that seemed easier. It's honestly hard for me to believe that taking a flight with a scheduled arrival of 10 AM into PEK is "risky" for a 530 PM connection. (Or an 840 AM arrival.) So a 7 or 9 hour connection is not enough? That sounds crazy to me, not that I don't believe you.

So, given that arriving the night before is not an option. Do you a
.) take the train or
b.) take the 630 AM MU flight,
c.) the 9 AM MU flight?

Thanks again for all the help!