As has been said, NeoOffice was created to make a Mac native (in look) OpenOffice. They do this using Java and some other stuff.
Sun then put their backing behind a full Mac port, and now we have the two.
Things to consider:
The OpenOffice guys asked the NeoOffice ones if they wanted to join, but they refused.
NeoOffice uses Java, which says it all really.
NeoOffice will always be behind OpenOffice in terms of general development