Yup take your time, I'm still doing some research about it, and won't go until later this year or early next year (although going there in winter is probably not a very smart thing to do). I'll be waiting for your thoughts.
I'll probably be traveling with some close friends, so we'll take turns to sleep and keep watch if necessary
I read up about the train journey; it's a new line called the Qinghai-Tibet railway, the world's highest railway line, with oxygen pump into the cabin. The ride is 14 hours from Goldmud in Qinghai to Lhasa in Tibet. It looks a bit of a runaround because I'll have to fly from Singapore to Xi'an, then Xi'an to Golmud, then take the train to Lhasa.
If I go by plane, it looks less complicated: Singapore-Chengdu-Lhasa. It'll probably shave off half a day or something too, but I'm worried about altitude sickness.
I'm also looking at doing Bhutan back to back, but I'll have to fly Lhasa-Kathmandu-Paro. I'll be spending 30% of my holiday flying all over the place!!