Personally, I am partial to northerns. From what I have heard, they tend to be slightly more docile, and a bit larger. I just absolutely love the colors of them! Still, you can't go wrong either way, just make sure to post lots and lots of pics of your new baby!
Also, just in case you haven't seen it, be sure to check out this caresheet http://bluetongueskinks.net/care.htm
By far the the most complete care guide of any reptile I have ever seen