Hi Nitin,
Suppose it depends on the version of your solution manager and managed systems.
If you have the latest solution manager and your managed systems are on a level that can support both ATC and UPL, suppose CCLM would be the way to go, as it has the decommissioning cockpit which has it's perks that CDMC doesn't have, such as waiting a defined time for an object to see if its used before deciding on it's decomissioning. If not CDMC would be a better choice as it leverages on ST03N to get the usage data.
In any cases, I have noticed one peculiar thing, CDMC and CCLM ad-hoc reports can seem to report different objects and different amount of custom objects(probably coz I am on SP10, maybe some other experts can enlighten us on this one).
Rishav has answered the other questions i suppose Image may be NSFW.
Clik here to view.
Solman stores everything for the case of CCLM.
Regards,
Chris
