It has been said several times already, but I have to throw in another vote for the Pentax 645N. It is just so much fun to use. The comparison to an 35mm on stereoids is perfect: fire & forget. It is very handholdable, AE works reliable and focus is adequately fast. My lens arsenal consists of the FA 45/2.8, FA 75/2.8 and the FA 200/4.0. The 45mm is indeed weak at the boarders as several reviewers already noted. But the 75/2.8 and 200/4.0 are astonishing. I also own the Pentax 67 adapter and it is fun to use those manual lenses on the comfortable 645N body.
The 645N II has some gimmicks to offer like additional imprint data, more custom functions which can be set at a Pentax service center, a different surface finish and mirror lock-up. They are all nice to have, but not necessary. I would go for an 645N II if the price is very good, otherwise go for an 645N, you won't miss anything.
For your interest, here is a comparison of the imprint data between both models: