NAS market is dominated mainly by 2 Taiwan based vendors QNAP and Synology. Since we sell both these brands, we are asked this question so many times - Which one is better. Now before you read on, remember that this is a never settling debate like Andorid and Apple. Both QNAP and Synology are excellent NAS devices and have a huge fan base and we are not going to explore which one is better. We will rather be shining lights on the various major aspects of NAS like hardware, Firmware and other capabilities of each device.
Firmware ( Operating system)
Both have their own patent Firmware based on Linux. Both are Graphics user interface that you access either through web browser or through Mobile app. Further both of them have App store through which you can download more applications to enhance the functionality.
Synologys DSM is simpler than QNAPs QTS. Other than that , both DSM and QTS are similar in function. Both have universal search so you don't have to dig through menus to find a setting you are looking for. QNAP appstore has slightly more apps than Synology , though not all apps are popular necessarily. QNAP offers the installation of QPKG applications which can be found on unofficial app store. With Docker support , both QNAP and Synology models which support Docker can easily install any third party app.
This is where QNAP outshines Synology not in the specifications but in the offering of assortment of models. Today Synology offers 40 models , and QNAP in contrast has 120+ models in nearly all the kinds of processors you can think of, for QNAP its easy to introudce new hardware through its parent company IEI.
QNAP also offers some hardware features that Synology doesn't - like HDMI port, FC Card connectivity ( which Synology soon is going to introduce) and 2.5"/3.5" Mixed Bay NAS. Choice of processors also is limited in Synology but not in QNAP.
On the Plus side, Synology models stay longer and planned for phase out gradually. Number of SKUs are less , making it easy for channel to Stock, so most of the time you will find all the Synology models available with the channel but with QNAP only a few fast moving ones.
Both the Vendors offer top notch Data Protection. Both have RAID, Pool, SnapShots, Remote Replication, Private Cloud Capability and Remote Public Cloud backup. There is not a feature missing from either of them so on this front, they both offer equally good options.
Virtualization and Containorization
QNAP offers a KVM based Virtualization Station on selected models , on which you can run a virtual machine. So Does Synology calling it a Virtual Machine Manager. Both are equally capable solutions.
QNAP offers both LXC and Docker for running containers. Synology offers only Docker which is the most popular Virtualiztion application today. In this aspect QNAP has a more complete offering if you have the need to run an LXC container. Typcially LXC is used to run the entire OS in a container whereas Docker is used for individual applicaiton.
Storage for Virtualization
Both vendors have vmware ready, Windows Server Certifed, Citrix Ready and OpenStack Comaptible models so both of them are equally competent in this point. Both also offer iSCSI on all the models so you can use both QNAP and Synology as an IPSAN.
Synology Uses Btrfs (butter FS). Contrary to popular beleive Btrfs is now a stable production release and offers many advantages to Synology. An HA cluster using 2 identical Synology NAS is one of the things that you can't do with QNAP.
QNAP uses EXT4 on some models and ZFS on the higher enterprise level models. On Entry level models, Btrfs is a better option and Synology has an advantage over QNAP.
Verdict - Both Synology and QNAP have their Pros and Cons
As you can gather from the above comparisons , there is no single winner here. Choose between QNAP and Synology depending on what features you are going to need. Both QNAP and Synology are very well reputed vendors with Solid product offering and deciding between two will eventually be down to your specific need and budget you have.