Fall is for Planting
Planting perennials in the fall avoids many of the adverse conditions that leads to plant stress and decreased establishment. The days are shorter, the air temperatures cooler and the soil temperatures are more suitable for rooting compared with the spring conditions (which quickly changes to the heat of the summer). As long as the soil temperatures are above 10C , the roots will continue to grow for most perennials. Good root systems in the fall gives the plants a great head start when growth resumes in the spring.
To me, the fall is clearly a better time of the year for planting perennials. Additionally, a well-established perennial in the fall will usually lead to a bigger, better display of color the following year.