There is no doubt that DataDomain is the market leading Dedupe appliance out there. They were close to first to market, and have lead the charge from there. That said, they have some of the best dedupe rates, but also the most expensive.
Then again, you should NOT dwell on dedupe rates too much. All the top 5 vendors seem to be <20% within each other.
If we use the 80/20 rule... If PureDisk gets you 80% of the way there, at 60% of the cost, can you give up some features?
One of the most compelling features about PureDisk, is that there is NO hardware agenda. DataDomain is expensive, and you are locked into EMC storage. Additionally, replication is included in the license, as is OST like functionality with NetBackup. All extra with DataDomain.
Currently DataDomain can dedupe NDMP from NetApp, but I hear PureDisk is getting that now or in the next month via a hotfix. Thus the gap is closing...
If you are stuck on an appliance, Quantum and Exagrid are doing pretty well in the market as well, and include the replication and OST license.