kram242 published a new build:

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In a lot of ways they do the same thing but differently. It really depends on the application your wanting to apply them to. Kinda like having just the right tool for the job. With V-Slots you obviously have the structure built in so you build it and you can ride a gantry on it. This is the main feature of V-Slot that makes it stand out in the world of modular building systems. There are other nice little pluses such as being able to run the belt and acme rods though the centers of the profiles as well as being able to connect and build on to it in unlimited configurations.
The OpenRails offer the unique ability to mount to almost any t-slot/v-slot system as well as sheet materials and basicly anywhere you can sink a a screw. For instance I has someone building carbon fiber drawers using OpenRail mounted to the draw sides and laminated into the layers of fiber I can picture this could be done with wood drawer sides as well. OpenRail also have the ability to stack one on the other while the 'V' overlaps on each side creating a single narrow rail that has a host of uses. One thing the OpenRails can do (that I have not see implemented) is bend around contours. Not sure what you would use it for, maybe a camera track? If you screw the rails to cut wood sheets the rails could potentially follow the curves and give an interesting linear rail. I suppose you could make an oval track as well. I hope this helps a little to give you an idea of just a few differences I can note quickly off the top of my head.
The best thing I can suggest would be to download a few of the models to see how both V-Slot and OpenRail are being implemented so far.
To be honest I think we have just scratched the surface of what can be done with each of these linear building systems and now that they are in the hands of the OpenBuilders community I look forward to an awesome 2014 machine design explosion of ideas!