When do you want to go in 2013? Different places are different in different seasons.
I would recommend China off the bat, I think there are more places to see and more scenic sights if you just want to go quickly in and out of cities. The places in SE Asia are more about spending time in the cities, beaches, looking at temples....enjoying night markets, etc. If you want to take a lot of photos, China is the type of place where you can see a lot of famous sights quickly, if that's what you want to do.

All of those places are changing rapidly and are safe. I can't speak to Cambodia, but I've never heard it's unsafe. Relatively speaking, Asia is much safer than other parts of the world (europe, south america, parts of the US, Africa...)

If you are interested in China, go sooner than later. We barely recognized places from our earlier trip, ten years apart.

I consider Cambodia very safe. Just take the normal precautions you'd take at home.

The place that is changing the most is Burma. If you are interested, go as soon as you can. But it would be a pity to move through it too fast - you would miss what is special about it. But two full weeks in Burma would give you a good sample. We were there in 2009 and again in 2011 and were amazed at how things are suddenly changing. There have been even more changes in the last year. I have two trip repots here, and there are now many others.

Thanks all!

I am not sure this helps me make a decision, but it certainly makes me feel that there are lots of good choices. I am going to look into Burma as well too.

I'll post again once I have a decision and a preliminary itinerary and look forward to feedback then as well.

Having been to SE Asia many, many times and having returned from China (Shanghai, Hangzhou, Lijiang, Yangshuo and Beijing) last April, a few thoughts on the flip side:

-China is not ideal if you want short transportation times between cities. If you're only going to Shanghai, Beijing and Xian, you're all set--but if you want to see more of the country and venture out west and down south, you're looking at very long flights in between places.

-If you're looking for anything "ancient" you're kind of out-of-luck in that department. Unfortunately, a great deal of China's true "history" was destroyed in the Cultural Revolution. If you happen to see something that looks old, you'll ask about it and learn it's a reproduction (yes, even of an entire town) probably dating back to the 1970s or 1980s.

-My experience in China was hardly one of things changing too fast. They've already changed. It's packed to the gills. It'll be even more packed in 5 years, so I guess if that's the kind of change you're worried about it won't really matter much.

As you tell, I wasn't a huge fan of China -- especially when in comparison to Southeast Asia. While there were a bunch of things and places I did like about our trip there, mostly I found China to be harsh, cold and lacking the reverence for its sights that a place like Thailand or Cambodia has...whereas I find most of Southeast Asia (especially the places you mention) to be accesible, easygoing, warm and very easy to cover tons of ground in not a lot of time.

Fodorites who love China: please don't crucify me for saying so...we really just did not enjoy our trip to China so I wanted to be able to provide a fair counterpoint.