Hi Marlon,
The 1131 is likely more "cosmetic" and is used in Office environments that do not require External antennas. The 1242 is more "ruggedized" and is generally used in harsher environments and areas that require External antennas.
We use many 1130's and have found them to be an excellent product. If you have any areas where the AP is likely to be exposed to high temperatures then the more "heavy duty" 1240 is the way to go.
From an excellent Cisco Q&A DOC;
Q. How does the Cisco Aironet 1240AG Series compare to the Cisco Aironet 1130AG Series?
A. Both series offer dual-band access points designed for the enterprise, both support interoperable 802.11i/WPA2-enhanced wireless LAN security.
The Cisco Aironet 1240AG Series is designed for greater flexibility. Dual antenna connectors for both the 2.4- and 5-GHz bands support a range of antenna gains and coverage patterns. The Cisco Aironet 1240AG Series is designed for more challenging RF environments, with a die-cast metal enclosure and operability in a wider temperature range than the Cisco Aironet 1130AG Series.
The Cisco Aironet 1130AG Series is designed for indoor office environments, providing rugged diversity antennas with omnidirectional coverage for both the 2.4- and 5-GHz bands. The antennas are integrated into the design, resulting in an unobtrusive form.
http://www.cisco.com/en/US/products/hw/wireless/products_category_buyers_guide.html#access_points
http://cisco.com/en/US/products/hw/wireless/ps430/products_quick_start09186a00805100f5.html
Hope this helps!
Rob
