You can see this article we wrote for more information. http://www.ciosolutions.com/Nimble+Storage+vs+Netapp+-+CASL+WAFL.
The main question is what type of disk is behind the SSD on the netapp 2240. Are you putting SATA or SAS disks on the netapp? But before we talk about that lets first talk SSD. Nimble doesn't pay a raid penalty on their SSD's where as Netapp's flashpool will be in a raid set and hence pay a penalty. However netapp's deduplication for a VDI workload will minimize the actual block footprint used in the SSD read cache(flashpool) so the raid penalty is probably not a factor. Basically from a read perspective it is a matter of which vendor's read caching algorithm you trust more to maximize read cache hit rates. Nimble seems to be pretty focused on implying their algorithms do a better job at determining which data to prefetch into the ssd read cache but I'm not sure how you actually measure that against netapp to determine who is right and who is wrong for VDI workloads. We have a customer running about 600 VDI machines on a netapp 3040 with flashcache and deduplication seeing 90% cache hits continually which implies that something is working right with netapp. We haven't gotten a nimble deployed for a comparable VDI workload yet to make a comparison.
In terms of writes this is where nimble is much better than netapp...unless you pack the netapp with enough SAS disks to keep up. We have seen first hand how efficient nimble's casl algorithm is at converting random writes to sequential writes ...which is how it can be backed by SATA disk and still get such great write performance. The netapp write performance is going to be entirely dictated by spinning disks...with two exceptions.
1.) The NVRam can soak up bursty writes...and drain those writes to the underlying disks.
2.) WAFL is pretty good a making random writes perform faster than the actual disks spindles will allow if their is sufficient "Holes". Sufficient holes means the disk doesn't have to spin much to get to the next empty location to write the block. The likely hood of sufficient holes reduces overtime which is why a good netapp rep will size the system to handle the write IOP using normal disk IOP specs even through the system is likely to out perform that due to advantages from WAFL.
This point often is not fully understood but the workload the kills a nimble is one where the random reads "working set" required in a given period of time is larger than what fits in SSD and drops the cache hit too low. That puts that SATA disks in a situation where they have to switch context from read to writes too frequently and SATA disk pays a latency penalty for doing this context switch. You combat this scenario with adding larger SSD. The reality is that it is very hard to have a workload that does this(we haven't seen it yet) and nimble does give you very easy visibility into your cache hits. The fact that you are looking at an X2 already means you have nothing to worry about for your workload and honestly the X2 may even be more than you need.
If it were me, since VDI has problems if write IOP isn't good enough...I would pick nimble. But Netapp isn't a bad choice if it is sized correctly and meets your budget needs. Netapp also tends to have more software features...which they license...but can provide huge benifits when needed. Netapp also has the best implementation of NFS which can simplify administration.
I haven't used tegile but they seem to be playing the we are the cheapest card which maybe isn't a confidence invoking message for some. They use ZFS which was open source from Sun before Oracle shut down the party....ZFS - Wikipedia, the free encyclopedia..So you would have to have confidence in tegile's ability to maintain the code they didn't originally write. My only advice would be to make sure you find some customer references that make decisions similar to how you would with an environment similar to yours. If you believe in the open source story and want to pay as little as possible then tegile seems to work.
Hope this helps.