The main difference is that Aironet 1100 Series APs operates at Data Rates of 1, 2, 5.5, and 11 Mbps whereas 1200 series APs have 2 typed of APs ( One is an AP with 2.4-GHz Radio and the second one with 5-GHz Radio Module ), the latter will operate at Data Rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
See the following URLs for comparison:
http://www.cisco.com/en/US/products/hw/wireless/ps4570/products_installation_and_configuration_guide_chapter09186a0080101c3a.html
http://www.cisco.com/en/US/products/hw/wireless/ps430/products_installation_and_configuration_guide_chapter09186a00801486a9.html#wp1006077
