Answer: Silver Queen Palms have a very fast growth rate once established so I'd go with the more economical smaller sized trees, give them a year to become established, and then stand back. They will grow quickly for you, and may end up being the same size in a few years. If you want a real difference in their heights, choose trees that are much more different in height when you first plant them than your suggested 2-3'.
Both aspens and eucalyptus trees will grow in your region. Aspens will need supplemental watering in the summer time; eucalyptus can tolerate droughty conditions once established.
Best wishes with your new landscape!
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