Application of visa outside through chinese embassy or apply in china?

Hi, my group of friends and I are intending to travel to china (Tibet, Xinjiang) for more than 15 days (less than 30 days). We are intending to get a guide to arrange our itinerary and accomodation only when we're at China.
Is it advisable to apply through the chinese embassy in our country in advance or apply when we extend our stay at China.
Thank you!