Hi Dorothy,
as far as I know, I'd say one big difference is that
EJB is a standard and Spring is not. Moreover I remember that the EJB specification allows another messaging service besides JMS for MDBs although I don't know what alternatives exist in practice. Because I don't know more details about Spring JMS, I unfortunately can't tell you more differences but perhaps this helps a little bit.
I want to implement JMS with features like load balancing, multithreading etc. and am confused over which open source software to go with.
What does this mean? Do you really want to implement a JMS provider or something like that by yourself? Or is it that you want to know which open source implementations you can use?
Marco