You've asked a difficult question for me to answer in a public forum. Each of these two products has a huge following (not to mention a huge company) behind them. Neither of them is the perfect product for everybody and neither product should be completely dismissed. As you mentioned, the biggest difference between them is that TSM uses an "incremental forever" methodology of performing backups and NetBackup uses the more traditional "occasional full and daily incremental" methodology. Both methodologies offer certain advantages and disadvantages. You just need to decide which of them you like.
The advantage to the incremental forever method is that you are only backing up files that have changed. You may therefore significantly reduce the amount of data that must be transmitted by the client across the LAN. This reduces the load that backups create on both the backup clients and the network. It does, however, create more work for the backup server. Without performing any additional work, the number of tapes required for a complete restore of a single system could number in the hundreds over time. Consider the impact this would have on your restore.
Tivoli realizes this that is why they offer two features to deal with this issue: co-location and reclamation. Co-location, if enabled, says that once a client has backed up to a given tape, only that client will be able to append to that tape. This reduces the number of tapes that a given client could be distributed across. (It also increases the number of tapes required in your tape library.) Over time, there will be "holes" within these tapes, as certain files expire and others remain. As time passes, these holes will also result in too many tapes being required for the restore of a single system. Therefore, you must use "reclamation," which will read all of the non-expired files off a certain set of tapes, write them back out to another set of tapes, and then expire the "holey" tapes. This reduces the amount of unused tape, but it obviously requires quite a bit of resources to accomplish. This is what I meant when I said that incremental forever decreases the load on your clients and network, but increases the load on your backup server. This means that you will need more tape drives and a beefier backup server than would normally be required for a traditional full + incremental method.
One real problem, however, is that many people aren't using these features well -- or they're not using them at all. They turn off co-location because it uses too many tapes, or they don't buy enough tape drives or backup server horsepower to complete reclamation. They're performing reclamation around the clock and the server's not keeping up with itself. At some point, these people then initiate a large restore, and they find themselves loading tapes for days. Yes, I said days. Therefore, if you like the advantages that incremental forever offers your environment, just be sure to plan for co-location and reclamation.
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