Both the trees you mention are cultivated varieties of Callery Flowering Pear (Pyrus calleryana). You would be better off selecting other cultivated varieties of Callery Pear than Aristocrat and Bradford. Bradford is susceptible to branch breakage in early and late snows that occur in Colorado. Aristocrat has a better branching structure, is more widespreading in habit, and has less problem with branch breakage in storms, but it does get some fireblight (bacterial disease).
Suggestions: Chanticleer is an upright narrow variety, as are Stone Hill and Cleveland Select. Chanticleer has performed well in Colorado State University trials. A variety that looks real good, but is not widely available, is Trinity. They produce profuse flowers but not much fruit set. Trinity gets to approximately 25 feet tall and wide, with a good red and orange fall color.
Invasives that are a problem here; tamarisk and Russian-olive have been placed on Colorado's noxious weed list. Pyrus calleryana has not exhibited tendencies towards invasiveness in Colorado.
