I think if you haven't experience with JBoss it doesn't matter which version you use.
But start with JBoss 5.1 is better because you start direct with the newer structure.
I migrate from 4.x to 5.1 without big problems.
A good way to beginn is to download the JB5.1 unpack and start it, this is easy.
You might start with a simple application (or a part of you app).
Most of the configuration (e.g. database) is described in the community wiki pages.
Also you can ask here ;-) and find help.
So if you have problems try to describe it with stacktrace, config files e.t.c.
and you will get answers here.
And BTW welcome to JBoss community
I will try a single module at a time and see if I can make progress that way.
Here is an example of the kind of robustness and error handling that seem to be missing in 5.1.0-GA: Just for fun I installed 4.2.3-GA and dropped in my resource files and EAR just as I was attempting to deploy them in 5.1.0. The 4.2.3 deployer clearly identified a problem in my XML descriptors:
org.jboss.deployment.DeploymentException: Error in jboss.xml for Bean AffinityGroupBean: resource-ref AffinityGroup found in jboss.xml but not in ejb-jar.xml
OK, I know exactly what is wrong. Same EAR dropped into 5.1.0 gets:
A search shows this is a known bug in 5.1.0-GA with no obvious workaround or even what causes it (I think this happens whenever there is a resource ref in jboss.xml that does not appear in the ejb-jar.xml, just as the 4.2.3 code shows). I was having to comment out large blocks of jboss.xml until the null ptr goes away, then start adding parts back to narrow down the problem resource. Ugh.
So I am thinking of getting the appliation deployed in 4.2.3 first where the deployment system seems to be more robust, then maybe try to take that to 5.1.0.
I think your right. I have some also some issues (in EJB2.1) where the error messages are a bit difuse.
But with EJB3 JBoss5 is better as 4.x, I suppose that the main development is EJB3 ;-)