How much do i have to step back in order to compose my picture?
That depends on how close you were to the subject in the first place.
Which lens is better and why?
It's really not a matter of what's better. It's just what you want.
So, yes, a 35mm lens on a camera with a 1.6x crop factor behaves about like a 50mm on a full frame camera, and 50mm is traditionally considered a "normal" lens. And a 50mm lens on the crop sensor will behave like a 80mm lens on a full frame camera. 80mm is typically considered a "portrait" lens because it lets you stand at a comfortable distance from a person and still fill the frame with their head and shoulders.
Put another way, with a 35mm lens on your crop-sensor camera and standing 10 feet from your subject, your (landscape oriented) frame will cover about 4.5 feet vertically. With a 50mm lens on your camera, and standing the same 10 feet away, you'll cover 3.2 feet vertically.
So, do you like to take photos that cover an entire scene, or do you prefer photos that are a little more intimate? Would you take a photo of a shopkeeper sitting in a doorway surrounded by crates of produce, or would you zoom in a bit and fill the frame with his aged face propped up by work-worn hands?
You won't go wrong either way, you might just have to adapt a little one way or the other by "zooming with your feet."
I am going on a trip to Thailand and will want to visit the night markets and temples.
Get both. You probably won't be going back to Thailand anytime soon. You can probably get both a 35mm f/2 and a 50mm f/1.8 for under $500. The 50mm f/1.8 from either Canon or Nikon is a well-known bargain, usually costing around $125. The 35mm costs more, especially if you get Canon's new image stabilized version, but a used non-IS version can be had for $250-300. So get both and have a great trip, and if you decide that you really don't need one or the other later you can always sell it.
Note that I'm talking about prime lenses here. The zoom lens that you have will obviously cover both focal lengths. The nice thing about the prime lenses is that they'll give you nice narrow depth of field, a sharper image, and more latitude for nighttime shots (no miracles, though).