I have toured fequently through that area, you will enjoy it, however the driving standards can be quite frightening. and you need to be alert to the crazy action that both Motocycles and car/lorries do. plus livestock wandering around the roads. unless you are planning on something like a month plus tour. As a first time visitor to the area I would say use one of the cycle tour companies. as they will take the paperwork hassles over for you I find Cambodia to be more primitive than Vietnam, in both countries the roads are improving, unlike me when I have been hauling myself through both countries you should for a 10 day tour look to stay within a small area of the chosen country. As to which of those countries to visit I would say do some research, read reports on Crazyguyonabike, see what routes the tour companies are offering.