I think it is going to be Laos. I dont think I can really do both Cambodia and Laos in the timescale I have available. My first thoughts were to try and atempt both but I would spend all day every day travelling with no extended stays in any one location to relax. Ankor Wat will have to wait for another trip (again!).
Its not very original but I think I will do the usual trip of Laos and take in Vientiane, Vang Vieng and Louang Prabang combined with a two or three day slowboat trip on the Mekong. Some of this will be backpacker central I know, especially Vang Vieng but I have always been a sucker for limestone karst scenery and some of the places I have liked most (Guillin, Halong Bay and Krabi) have a similar jagged teeth geology.
The rough plan is to arrive at Don Muang and either fly diectly up to Udon Thani with Air Asia or Nok Air and then cross over the Fellowship Bridge at Nong Khai or take the overnight train up to Chaing Mai and then bus it to Chiang Kong and enter Laos at Houayxai.
