While Tehran is the capital and administrative centre of Iran - both current and historically - Shiraz is the hub of Persian culture and has been for more than 2000 years! Shiraz is also synonymous with poetry as the city is the birthplace of Persia's greatest poets - Sa'adi and Hafez. The best time to visit Shiraz is spring time when the numerous gardens and parks in the city are blooming with a variety of flowers and roses.
The best time for visiting shiraz is April and March .when you can smell the orange blossoms and enjoy the pleasant weather. If you like to have an experience of Iran and shiraz, please visit our itineraries : http://www.irantourcenter.com/tours/1/iran-cultural-tour/default.aspx
This historic city is very manageable next to Tehran -- walkable and there is comparatively little traffic. The main sites are all within walking distance of one another. Far from the capital, this isn't a religious center and the more relaxed and liberal vibe is immediately apparent just based upon how people are dressed.
Shiraz, highly recommended for any trip to Iran, Shiraz is a easily manageable heartland of Persian culture with most welcoming people and Persepolis and Pasargadae as its Architectural gems, for tours to Shiraz: http://www.irtouring.com