single or multiple entry type?

My husband (Swiss Citizen) and I are flying into Beijing, staying 45 hours then flying to Mongolia. About two weeks later, we are getting the train from Ulaanbaatar to Beiing, travelling in China for two weeks before flying out of Shanghai.
Since we are staying less than 72 hours on our first visit, should we just apply for a single entry visa for our second stay (and therefore valid only 3 months)? Or should we apply for one entry permit for two entries?
Many thanks