Tourist Visa - L vs. Transit Visa - G

If my husband is traveling to Mongolia with a likely stop-over in China, would he apply for a transit G or a tourist L? If he gets a G, it stated that a visa for the country of destination is required along with an airline ticket. Since Mongolia doesn't require one, he wouldn't have it. Is that a problem? Also, how long is he allowed to stay in China with a transit G? He will be traveling through China both going and coming from Mongolia.