not sure where you are going with this (trying to make Haradrim Cavalry?), but you definately would want to use the Riders of Rohan over the woodies.
Not only are they cheaper, but they are the correct scale. The WHFB wood Elves are huge compared to your other LotR miniatures, and the wood elf horses are way out of scale, and look, quite frankly, horrid. (leather masks on a horse? On half of its face?)
any reason you are not going to just buy the cavalry? Trying to save money? It would be cheaper to buy the Riders of Rohan plastics and replace the riders with the metal raider figures than to buy the haradrim raiders. That would cost about $10 us per figure, and you would have a small pile of plastic riders to use for other conversions.