The X400 is, for obvious reasons, going to be better for a pistol application than a long gun. Not only is the form factor better suited to that application, but the beam is "optimized" for handgun use. This is not to say that pistol lights can't be used on long guns, I've used X-series lights on long guns before as well, rather just to be aware of the natural limitations of using the device outside of its design envelope.
Sadly, the Unity EXO is not compatible with the B-series attachment format, which does a great job of protecting the activation tabs.
As far as comparisons to more dedicated long-gun units like the DBALs or ATPIALs, it depends in part on your desired application, LED illuminators will often give you better performance than laser-based ones at the Class I level, but with a visible signature, while the X400V specifically will give you a taller unit with more offset, and a much more exposed switch in pretty much all aspects. Moreover, having a combination light/laser unit also means that you will not be able to transition between NV and white light engagement as quickly as having a dedicated illuminator and dedicated white light.
~Augee