Hi Imran,
The main difference between the two is the Cisco WSA is an appliance or a hardware while proxy server is a software. Other features are also offered by each security solutions. Best recommendation depends on what features are you looking for. Kindly email me (alyreyes@cisco.com) for more information and so I can also provide you options that you have.
Hope to hear from you soon.
Regards,
Alyza