kansui powder vs. lye water, which one is better for noodles?
This is a little tricky to answer because of terminology.  Kansui powder is a mixture of potassium carbonate and sodium carbonate, in powder form.  It is typically added to a noodle recipe in a range of .2% - 2.5%, depending on the type of noodle.

When it is sold in liquid form, it is also known as Kansui, but may be labeled as "lye water", or "alkaline solution", or with the ingredients "potassium carbonate" and "sodium carbonate."

They are both the same thing, and you can turn the powder into a liquid by creating a solution. 

The terminology issues arises because this product is sometimes confused with food grade lye (sodium hydroxide), which is used, for example, in German pretzel making (among other things).  They are different products, with sodium hydroxide being much more caustic, and potentially a safety risk if not handled appropriately.