I've done the backpacking routes with everything in one big bag (70L, big enough to carry a friend for free) and with 2 smaller bags (about 30L max). The second option was the most flexible, comfortable and easiest to control. I had to repair my new P&S that was in the 70L (admittedly I could've packed it better) as it got flung around a lot at each airport.
With 2 smaller bags you get to keep one as your carry-on (and thus your camera, small change of clothes, money and stuff you can't lose) and the other takes its chances in cargo. It's easier for getting around on public transport this way, one bag on your back and the other on your front.
Best of all, with 2 smaller packs it's harder to overpack, and since you're heading to Asia, you don't need to bring much clothes-wise - cheap and easy to buy stuff locally, and you won't feel guilty ditching it when it gets worn or you don't want to pack it back home.