I have been with only American women and Asian women, and I prefer Asian women, personally. Several reasons, but only 2 of them are purely sexual. One is a physical difference, they other is cultural.
Sexually:
1) Something is different about the way their knees or hips are built, or something. Every Asian woman I have known can squat for long periods of time with no discomfort. If they need to do something on the floor, like wrap a present or roll up a sleeping bag, they never get on their knees- they just squat. Sexually, this makes the woman-on-top position better.
2) Culturally, and this one is likely to vary a lot depending on how westernized they are, the Asian women I have known deeply feel that it important to keep their husbands happy. Its a point of pride to them. They are competitive about it. They would be ashamed if it got out that a husband was unhappy. This applies in the bedroom and out. My wife does ALL housework, lays out my clothes, etc. She is not the slightest bit submissive, either- she frequently tells me what to do- but she really tries to do nice things for me all the time.
Another thing I like about Asian women: I know like 15 Asian wives- The average weight is probably 110 lbs. The Average American woman weighs 160 pounds.
I have had several relationships with American women. Many of them were good fucks, but generally, I felt that none of them thought my happiness was particularly important. I just want to be treated nice.