If your application can use either SIMPLE (you have a working SIP stack) or SOAP, then probably you can go with whatever is easiest.  Some possible considerations:
 
SOAP
------
- Fairly easy development using WSDL auto-generated code
- Easier to debug/troubleshoot: retrieve readable logs vs trying to get a network pcap
- Exposes additional features, including management/provisioning APIs
 
SIMPLE
--------
- Significantly less performance overhead, greater scalability
- Standards compliant
- Limited to core presence functionality

My understanding is that the CTI gateway interface (CSTA over XML:  ECMA 323) is intended for use with the MOC, and not really for general 3rd party use.  Will check on this.  At least for now there doesn't seem to be any detailed documentation or a WSDL published.
 
For your purposes, then, I would suggest a standard SIP line-side integration directly to UCM
 
http://developer.cisco.com/web/sip/docs
 
or possibly using TAPI/JTAPI CTI directly to UCM:
 
http://developer.cisco.com/web/tapi/home
http://developer.cisco.com/web/jtapi/home
 
Note, you can also subscribe via SIMPLE direct to UCM, though this will be line-based presence rather than CUP's user-based presence
 
 

Hi,
 
Could any one help me knowing that how the users are stored in CUP Server (are they stored in a DB maintained by CUP or it internally make use of Active Directory of the comany or enterprise for which it is configured)?