arbor makes two elements one is cx one is rx. If you missed it before the rx is rocker and cx is your traditional board design. reg camber will be more stable at higher speeds where as rocker will give you quicker tighter turns both boards can do either they'll just feel more at home doing what there designed for, though that wasn't your question.
The difference in the Coda compared to the element rx would just be the rx will be a bit stiffer, and quiet possibly a bit better though id have to ride it to be sure
a good rule of thumb to go buy keeping everything in mind of course the more expensive a board is within one brand the more materials and therefore a likely better candidate for big mountain riding, however there are always execptions to rules.*cough*burton*cough*
I just looked at the arbor site again, the only difference I can see between the element rx and coda is the element has mystic core 1 and coda mystic core 2. Every other spec and feature is the same. And the descriptions on the cores are so vague you don't really know what makes them different.