welcome to the forum.
If you mean JBoss AS6 vs. AS7 there is a big difference.
AS6 is a follower of AS5 but not longer developed.
AS7 (now known as WildFly) is (mostly) a complete re-written server. It will be faster and easier to configure.
The current development is here and new JavaEE specifications will be available here.
So if you start new with JBoss you should start with AS7 or if you have a longer schedule with WildFly which is the current and will be released in the near future.
If you start with AS7 you need to build the server from source as the latest version is not available from the download page.
If you need a long supported, stable version you can try EAP6 (which is based on AS7) but you need to buy a subscription if you try to run this version in production. But it is supported for +4 years.
Thanks a lot Wolf-Dieter Fink . Can you provide me with some more input regarding JBoss7. Can you point out the features, advantage and disadvantages of it. And what are the reasons a developer thinks its a suitable server for them.
Thanks a lot.