You may already know that Cisco has published End-of-Life (EoL) announcement for many of their WLC platforms as of 10th October 2016. This includes WiSM2, Cisco7510 and Cisco 5760 controllers recently. 10th April 2017 will be last day of purchase these products.
You can find related EoL announcemnt for each product shown below.
So if you already planning to buy any of these models, you should look for alternate products. You have limited choice to select from either 5520 or 8540.
I am sure 5508/2504 will follow the same path soon. So plan ahead your WLC replacement projects.