I'd say that there are some considerable differences between the NWT and NNWT, so much so that I think of the NNWT as Jehovah's Witnesses' GOOD NEWS bible.
In actuality the NNWT (despite its ongoing online updates) is really the third generational version Witnesses use. The first being the green editions, the black second version and what we have today ... being the grey/silver one.
As we know the NWT was originally released as a series of volumes from 1953. And I do consider them to be a rather good 'word for word' translation barring the decision to insert the word JEHOVAH into the Christian Greek scriptures, where there is clearly no authority for them to do so. In fact it is clear that the use of JEHOVAH in some verses was inserted to "clarify" or shape their meaning, which runs foul of REV. 22:19.
The second iteration was released in 1984. This was the first leather bound, traditional looking Bible, which Witness took into the ministry and honestly, it something that many took pride in using. It was well bound and had a easy to read font. However, the unfortunate tradition to include the use of the word JEHOVAH was only expanded in the New Testament.
Finally, we have the third attempt by the 'Faithful Slave' to produce a copy of the Bible, offered in 2013.
While some would say that it was easier to read (which I doubt its relevance, as I still consider the 1611 King James as quite understandable if you want to put the smallest of effort into it), the publishers once again decided to move further away from the word-for-word concept of Bible translation and go with a more looser text. This allowed them to make modifications that colored it's wordings to reflect Watchtower doctrine. Case in point, is the word SOUL, which has now been almost completely removed from the NNWT. Another example can be seen is the revisions and inclusions of words to reflect the concept of a 'governing body' in the first century. To my mind, this is inexcusable for people who claim other Bible translations, as tainted.
In summery, the NNWT is a poorer version, for those people who wish to have a translation that attempts to reflect the original manuscripts because it has shown over time, to value prose over textual accuracy.
Of course it only my opinion ... and who am I? Not spirit anointed Governing Body member with an expert handle on the Hebrew and Greek languages, that for sure.