Coincidently, I am facing the same issue. Recent heavy rain worked its way behind the plastic rock guard through the night and the weight ripped half the plastic off. After looking at a number of options, I finally settled on Line X (will be applied next week).
I wanted diamond plate initially but two things caused me concern - price (about $1000 Can for a 7 X 3 foot section) and thickness (it was way thicker than the original plastic and that meant the trim would have to be removed all the way over the top of the trailer as it could not be just slipped under the current trim - too much work). If you do go diamond plate, make sure you confirm it is the real stuff as some trailers use the plastic look-a-like.
I have seen Line X on other trailers and it looks quite good. It is guaranteed and it only cost $300. The only issue I am having is taking off all the old glue (although the surface will be sanded prior to application). I am also considering getting the tongue and frame Line X'd for an extra $150 as it it starting to show some wear and rust.
Marv