I have always shown my Arab in romel reins. The way weighted romels droop looks very nice with breeds that arch their necks. I would recommend not using them on stock horses, they wouldn't lay as nice. For the stock breeds, definitely go with flat, split reins.
I agree with Caitlin. When I showed arabs I used them and quite liked them, but they look a little out of place on most stock horse breeds.
That's what I always used when I used to show western pleasure. I like them a lot on light breeds like Arabs or Arab crosses, morgans, etc. But I definitely prefer to see split reins on stockier breeds like quarter horses and paints.