You're building a house. Which do you do first?
Both approaches have merit. If your heart is set on a Spanish style adobe home, a heavily treed lot may not make sense for you. Having an idea of the architectural style you prefer will determine the size and characteristics of your building site.
You may run into problems, however, if you select a specific floor plan too soon. You can always design a home to suit a landscape, but you may not be able to alter a landscape to accommodate the specifications of predetermined house plans. The configuration of rooms, the placement of windows, the location of the driveway and many other design elements will be affected by the land you build on.
The land itself has long been the inspiration for truly great homes. Consider Frank Lloyd Wright's Fallingwater. Constructed of concrete slabs, the house is anchored to a rugged stone hill in Mill Run, Pennsylvania. Compare Fallingwater with Mies van der Rohe's Farnsworth House. Made almost entirely of transparent glass, this unearthly structure seems to float above a grassy plain in Plano, Illinois.
Would the Farnsworth House seem as graceful and serene perched on a rocky hill? Would Fallingwater make such a powerful statement if it sat in a grass field? Probably not.