Really depends on what your needs are and what system you perfer working with. We currently use both. The GUI interface of Netbackup is easier to use and navigate to me as far as administration from the server side. I know next to nothing about working on the server side of TSM, we have another admin who does that. Installing the netbackup client is far easier in mho as well. However as far as performance and reliability goes TSM is doing a far better job and has better options. It just takes a lot more planning and configuration than netbackup does.
Is there something more specific you would like to know?