SoundOff's nForce series edges the Impaxx6 slightly, IMO. I think it's optics spread the light much better while maintaining forward punch. Another brand's rep saw them in person for the first time yesterday during a visit and was impressed with their brightness. That's no faint praise. They also have a large "light footprint," by which I mean that from a distance, they appear to be bigger lights than what they actually are, if that makes sense. I like that in a LED light, because I think the perceived visual light footprint is just as important as perceived brightness.
Also, IMO, the SOS is easier to program and sync, but that might be a matter of preference/familiarity. The SOS unit has a nice finished look and appears to be better constructed. Plus, they still ship in a box vs. a bag and I just like that. Having said that, I certainly wouldn't be disappointed to have IPX6 units, and we recently put some on a slicktop. Pricing is pretty well the same and both have 5-year warranties.