South Dakota is king, period. But, early in the season you can't hunt before noon, then 10. It's a drag sitting around all day. Lots of places you go will have so many birds you'll be done in a couple of hours anyway. That being said, it's a cool experience. It can be expensive.
Western Kansas is far less pressured and has a great deal more public land. The WIHA program is much more successful here than in SD. The major difference is that SD has so many more birds. There isn't really a comparison. You'll see dozens of birds just driving down the road in SD (which you can hunt because SD allows road hunting). The good part about KS is that you can do it more economically.
Let me put it this way:
Here's a 2 hour hunt we had in South Dakota 2006
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And here's a whole day in Kansas 2006
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Obviously it depends on the day and the place... The very best pheasant hunt I've ever had was in KS, but on average SD is better.