You only have 2 weeks--do the UA flights! You will be covering a lot of ground and trains & buses take a lot of time. Or do Air Asia if that works out cheap enough--you can book online. I've taken trains in Asia and while they are cheap and fairly comfortable, they are not all that fast. And please do NOT do overnight buses! Oh my God, I took an overnight bus in Vietnam and it was the worst travel experience ever--like riding in the back of a cement mixer for 8 hours! I'm a budget traveler but sometimes it's not worth saving $20 if it takes twice the time.