Depends on how picky you are.
Both work the same if your goal is to keep mud off your car. From what I've seen, rokblokz tend to warp over time- the fronts bend towards the wheel from the little stock plastic end piece on the side skirt pushing them forward, and the rears backwards from the wind or something. RallyArmor has better mounting hardware/system and stays straighter/looks better IMO.
I personally got the RA ones, but if your'e just worried about keeping mud off your car in the winter, i'd get the RB and spend the 60 bucks on beer.