The dominator RAM is tested to higher standards and designed for systems that are going for higher overclocks. Generally it is supposed to be more stable and able to take higher voltages. Other than that, there isn't much of a difference these days.
For myself, I tend to use Vengeance unless I absolutely need to wring every last MHz out of an overclock.
The pricing is stupid, I bought mine admitedly for the looks. I have always loved how the Dominator series looked. Not gaudy, just slick. Although other companies are catching up in the area of design.
The RAM is great though but I at the end of the day unless you are looking to OC like Crazy then I think Vengeance series RAM is the better choice
Vengeance LPX is our mainstream DDR4, utilizing a standard height PCB and heatspreader. Dominator Platinum DDR4 adds a larger, more robust heatspreader as well as compatibility with our Light Bar Kit, Dominator Airflow Platinum fan, and Corsair Link for monitoring voltage and temperature (Airflow Pro required.)
Corsair Vengeance is Corsair's mainstream line of SDRAM modules. They're designed for power users who don't want to do too much tweaking or fiddling; these modules are designed to just work. To this end, the specifications on the modules are fairly standard and are almost always marketed with JEDEC standard supply voltage (1.5 volts) and standard timing settings. Note that Corsair Vengeance Pro is a separate lineup that is more in line with Corsair Dominator.
Corsair Dominator is Corsair's enthusiast line of SDRAM modules. T hey're designed for enthusiasts who want the best performance. These users aren't afraid to spend a few hours tweaking their PCs and are comfortable dealing with stability issues as they arise. Dominator modules are available in non-standard supply voltages (such as 1.65 volts), tighter than usual timings, and often come with exotic heat spreaders and cooling assemblies.