There have been lots of discussion about this already. Have you tried searching the forum?
They are different models with different specifications eg Engine power. Pop into a Nissan dealership and view the cars, that's what i would do after doing a search.
Have a look at the "D40 BIBLE" in the d40 section of the forum. It lists just about everything you need to know. I think its a sticky, and also do a search as im sure this exact topic has come up quite a few times.
As far as the engine goes it will depend allot on what year models you are comparing.