Hi, seeing your experience I think you should start up from SCJP. As it is mandatory for SCWCD, SCBCD, SCJD. I would say go for 1.5. 1.6 is very fresh and new, and I doubt if there is any exam released for it yet.
Hoo hoo hoo! Looks like we got a live one! Here, wave this tiny ad at it:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop