How would you compare Dell PowerProtect DD vs NetApp FAS series?

Data Domain is designed for a backup solution and it is capable of deduplication and data compression where NetApp and Pure storage arrays are used to store data.

If you are looking for backup purposes you can compare with Data Domain, ExaGrid and Rubrics.

This appliance is designed for a backup purpose.

@Dhruba Roy, your question conflates very different kinds of storage. 

PowerProtect DD is Dell's latest version of Data Domain. It is ONLY useful as target storage for backups. Nothing else, not even archiving. If that is what you want, it does what it's supposed to do. Albeit, it's a bit pricey and underperforming. 

There are much faster, cheaper, and more advanced backup target storage. Especially when measuring restore performance. I would suggest you take a hard look at a variety of backup target storage vendors including, Infinidat InfiniGuard, ExaGrid, Quantum, StorONE, iXsystems, and many more. Most backup target storage is all HDD although some are hybrid SSD and HDD.

NetApp FAS is a general-purpose storage system for blocks and files. It can be all HDD, hybrid HDD and SSD, or all SSD (all-flash FAS or AFF). It's a solid all around storage system with NetApp pioneered capabilities, but expensive as a backup storage target. 

Pure Storage FlashArray//X or //C are block all-flash storage arrays. Their FlashBlades are all flash file and object storage systems. Good performers but overkill and way too expensive for backup target storage.

I think you need to define what it is you really need. Of the 3 vendors you asked about, I am going to repeat myself, PowerProtect DD is ONLY useful as a target storage for backups. The other two can do so, but are really not priced nor designed specifically for backup target storage. 

If general purpose storage is what you need NetApp and PureStorage are good possibilities among many others.

I think they are different types of storage for different purposes.

If you are looking for a storage where to put backups data you can think Data Domain is the perfect choice because it is its main use (most or all the backup softwares have plugins in ordere to manage data domains).

If you are looking for a primary storage (where to put your servers' data) then you can look to Netapp FAS and Purestorage.

The latter are flash natives so it's simpler to manage and configure. If you look at the Netapp FAS you can also choose storages with HDDs with less performance (and a cheaper price).