In CCTV systems which is preferred ?

it depends on how the costumer looks for :

if customer looks for more robust and minimize  the effort of integration and compatibility issues between different system for sure he will use one solution appliances from one vendor

if customer looks for totally expandable system with integration of different technology and upgradable to state of the art technology of course will choose open plate form and storage system

Storage should be open platform but the application must be dedicated.