I would definitely choose Myanmar/Burma. Avoid the well beaten 'banana pancake paths' in Laos and Thailand especially in mid March which is part of the peak season. Of all SEA countries Burma is the most interesting for a photographer by far. Been there twice and would definitely go back again. Get a visa in Bangkok, Khao San area (2 days normally) and fly to Yangoon. Then again I would advice not to take any flights within Burma. It can be unsafe. Take the (really badly) rattling train or a bus instead and concentrate on Mandalay+surrounding areas and Pagan. Someone will keep an eye on you (probably your friendly taxi driver/guide) so don't try to shoot anything politically compromising and everything will go smoothly.