DataCore vs. Nexenta vs. Tintri - which should we choose for our exclusively Citrix shop?

It Depends, tell us more info on your needs, requirements, why you are looking at just those three which represent three different approaches, or, did you look at three (or more) different approaches and these are the winners of those categories? How many virtual desktops will you be support, what is their workload/applications profiles and related info? If you are looking for turnkey converged with server, storage, basic connectivity, hardware and software all in one solution then of the three that would be Tintri. However if that is the route you are going, then what about Nutanix, Simplivity etc... Otoh if you are just looking for ZFS based software solution for san/nas etc to be deployed on different hardware, there is Nexenta as well as CloudByte, SoftNAS not to mention other variations including from those such as StarWind, OpenE etc... If you are looking for storage virtualization, there is Datacore which has been around for awhile, however there are others including the ZFS bases, StarWind, OpenE, EMC ViPR and many others...

So knowing what the problem is that needs to be solved will yield the answer of what is applicable or best for that given scenario.

There is no straight answer on this topic. Each storage architecture is different. Since the whole premise of SDS is to abstract the storage management form the hardware, Does the vendors software provide management for all your storage hardware brands and also all types of storage you use? Does it meet the end users needs and IT's? Assuming that you have a list of got to haves and nice to haves, all you can do is compare them. If Datacore meets your needs and you and IT have done your due diligence then go with that. There is always going to be someone with something "better".