What are the differences between each driver? I mean, besides one of them being open-source
What are the pros / cons of each one?
Which one would you recommend?
here's jTDS own opinion on the subject: http://jtds.sourceforge.net/
They are distinct implementations. The jTDS driver is a type 4 and in my experience has better performance. I have been using the jTDS driver in a production environment without any issues for 5 years.
I would recommend going with jTDS.