We had a "Rock Guard" (That was the name brand). It had 2 inch flaps that hung side by side. I also had mud flaps on my truck.
My current set up, with my Equal-i-zer hitch, I cannot attach the Rock Guard, as the hitch shaft is made different. But my current truck also has mud flaps.
Have a noticed a difference in the front of the trailers between the 2 scenarios? No. I think the mud flaps provided the best protection. My current trailer, hitch, and no rock guard ... well ... after 5 years and thousands and thousands of miles towing, I don't have any nicks on the front fiberglass cap of my trailer. My previous trailer with the rock guard had diamond plate across the front. I don't think it ever had any dings either.
Personally, mud flaps provide the first line of defense protection for your trailer. Rock Guard flaps are secondary. If you have no mud flaps, then the Rock Guard would be beneficial. But get the solid one. Not the 2 inch danglie ones, or the ones that remind you of the straw on the end of a broom.