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For most of the trip, there won't be much to see except the expanse of America, and both sides are probably about the same. When I fly from Dallas to Denver, most of the time the plane heads north along the Rocky Mountain front range once it gets into Colorado, and in that case, you want to be on the left side of the plane for the view of the mountains. But this can change based on wind direction and/or weather.

In the morning and late afternoon, the sun will be out of the left side of the plane as you fly west. This can be a negative if it happens to be shining right onto your face.