In my opinion, the diamondplate repair would give you a better appearance than what you'd get by injecting e-poxy, but the e-poxy injection will give you a better structural repair.
When you cut out the damaged material you'll be removing the structural components of that area. The diamondplate won't replace that structural strength.
When done properly, e-poxy injection actually penetrates the damaged wood fibers and replaces the lost wood strength with new fiberglass strength. You would be surprised by how much strength you can restore through this method. Unfortunately, when you're all finished, you will see the repair "holes" where you injected the e-poxy. Unless you're an expert at body work.
For my money, I'd go with the structural repair over the diamondplate.
-Speak