I think for the most part, the differences in Gen, is mainly ease of reading. The New Jerusalem bible is in modern english and the KJV is in old english. The main differences in the 2 bibles are that the Catholic bibles have a few more books in them as well as in books like Daniel have an extra chapter, others have a few verses (considered the apocrypha) that are not in the KJV, or any other protestant bible. When it comes to what it says, I would rely on the catholic bibles more then the Protestant ones, because the Catholics are the originators of the bible, first translated it and also have the oldest manuscripts.
Mainly it is just the wording. However...There ARE those Christians who claim that the KJV is the ONLY "authorized" version, and that any other version is basically a "perversion" of God's word and therefore should not be read. If you are surrounded by those types of Christians, then you have my sympathies.
The KJV is really hard to read and then understand. Today our English language does not use thee or thou etc. a New World Translation is just that an understandable English bible. Not changed in context. If you read just one chapter in the KJV by the time your done you have read so many thee's, thou's that you don't know what you read. when the original bible was written it was written for the people of that time. It has evolved to other languages plus English. now you can actually read it and understand what your reading.
I don't know about christian bibles in any specific way, but I do know that " wording" is not a petty concern. just a few different, and seemingly unimportant words can change. the meaning dramatically. it's always prefferable to have an exact copy of any document or writings.hope this can be of some help.
I love the NJ bible. It's much the same as Genesis in KJV. The difference is it contains the Apochypha & contains Gods Holy name all throughout. The book of Wisdom & Ecclesiasticus are amazing, they are a must read. :-)