JBoss Remoting is built into JBoss AS (at least the newer versions), so will be there by default, so won't have to worry about plugging it in. My guess is that JBoss Remoting will be faster than JXTA for typical invocations (based on most JXTA implementations being XML based). I feel confident that the API and configuration is easier to deal with than with JXTA. Can also very easily switch out the transport protocol being used with JBoss Remoting.
However, JBoss Remoting does not currently support multiple endpoints for invocations (but is on the road map). I'm not sure about the autoconnect/disconnect features of JXTA, so am not really sure about what this is or how it works. JBoss Remoting does require explicit connecting to remote endpoints (when using stateful transports).
Hope that helps.