JBoss Dev Studio (also known as RedHat Developer Studio) is a branded version of a Eclipse packed with the right version of JBoss Tools. The dev studio is usually bundled with a developer subscription from RedHat for $99. (I’ve been told, however, by Max Rydahl Andersen a JBoss employee who was also one of the developers of JBoss tools and the developer studio that the JBoss Developer Studio IDE itself is actually free without the JBoss commercial runtimes – i.e. JBoss EAP).
If you stick with Eclipse IDE with the JBoss tools plugin, I came up with the following matrix of the set of tools showing the coupling of compatible versions.