The tri x has a better cooler, and may be clocked a little higher. If you don't mind overclocking it yourself, save the money and get the dual. It should easily hit the clock speed of the tri x anyway.
The tri-X costs 30$ more, is it worth getting it over the Dual-X?
Tough call. I had the same dilemma and chose the Dual-X. I just didn't see the value in it. Also, I haven't seen any reviews of the Dual-X compared to the Sapphire models. Really you are comparing cooling ability and noise. I think this was one of the reasons I went Dual-X.
http://www.eteknix.com/sapphire-amd-radeon-r9-280x-vapor-x-oc-3gb-graphics-card-review/14/
Vapor cools much better but that noise.