We have a customer who has a very large environment, and the big question now is that as part of the migration to NBU 6.5, should we use VTL or Disk Stores? The choice will be almost purely based on raw performance (MB/s) in both backup as well as restore. We shall be using NetApp either way - no choice there at this stage.
Just remember that PureDisk can dedup with regular storage disks not on your VTL. SO if you are paying a TON more for a VTL than just storage for netapps deduplication, you may want to consider just buying the puredisk licensing instead which will probabbly save you quite a bit. Also, if they have exchange, and you want to do granular restore (this is VERY nice for any exchange admin) then you'll want disk storage as that is currently the only way for performing granular restore.
Since people are moving to disk storage as the future, at least for development (take for example, synthetic backups perform better on a disk storage than a "tape mount" from a VTL), you may want to consider being in that range.
However, If netapp is offering something amazing on its VTL that is a "Must Have" then you may want to consider that.... i know for example that alot of the SAN configuration is easier to do with the VTL in my past experience).
VTL only writes in thousands of megabytes per seconds because of the way the disks are striped and contained. Disk storage can do the same thing, especially if you are using SAN storage. Please don't "assume" one is better than the other, they should provide you read/writes for both if that is the case. Remember, you can only go as fast as your slowest item- so if you have a slow media server, or a capped out network, then theres really no point of a "much faster VTL" versus something that is more reasonable.