Next year two of us are planning to visit Nepal and Tibet, using KTM and Beijing as our entry/exit points. We hope to cover the route by train where possible. I am just looking for advice on which way is easier/less expensive.
Tibet -> Nepal
or Nepal -> Tibet
Thanks in advance.

Tibet -> Nepal. No doubt about it. Entering the other way means dealing with group visas, and the hassle of getting them changed to a regular visa when leaving the Tibet region. If you start in Beijing, you can do the entire trip on your Chinese visa. Just make sure you have enough time on it when you get to Tibet, for it is very difficult to get an extension there.

Hi,
If you just want to visit tibet , you do not need have a China visa. The tibet Visa can be issued in Kathmandu, but you can not visit other part of China. Kathmandu---- Lhasa(take flight ), Lhasa --- Kathmandu( take landcruise)
If you wanna to take train to Tibet, using Beijing as your entry points. TTB can be issued by tibet tourism bureau, you should ask a travel agency to arrange for you.
You should pay attention to the weather, the road condition in some remote area is not very good, do not visit Mt.Everest on August .

I cannot recommand Nepal-Tibet. I did it years before and had to be part of a tour. This time a organized everything myself. Try to get to Tibet and to organize everything in Lhasa. There are many agencies going to the nepalian border. (and it is cheaper this way. Don't trust agencies in Nepal, they are only interested in their profit!)

Tibet to Nepal, saves you a lot of hassle. And you DO need a chinese visa to enter Tibet from Nepal (#2), it will be as groupvisa, normally valid for 21 days. Can some places in China be changed into a regular chinese visa but no guarantee.
