I believe seiza is easier for Japanese people for the simple reason that they have done it all their lives since its part of their particular custom. Just as an example, my sensei told me that when Chida sensei came to North America and they went out for dinner he sat in seiza on a chair. It was too awkward for him to sit in a chair the way most North American, European, etc. people would.
When you first start going into the seiza position it will be difficult and become quite uncomfortable at first, but it you will become accustomed to it after a while. One thing that you should keep in mind when doing it is that you are not suppose to sit on your feet, but you should keep some tension in your quads and try to maintain a thin gap between your upper and lower leg. The reason for this is so you get up or respond quicker to an attack. I think its a bit more comfortable like this once you get used to it. Ive been told that you should be able to sit in seiza for 30 minutes before it getting too unbarable.