Hi Nick
The H4D40 is the latest technology and the High ISO performance is very impressive IMO for medium format, it also has true focus which is cool. There are quite a few posts about it on my blog:
http://www.nick-t.com/blog/tag/h4d40/The H3D39 is a physically bigger chip which might matter to you if wide angle coverage is important.
As for the focal length I do love my 28mm but I'll leave the landscape chaps to answer as to which they prefer. Be sure to read what Derek and others have written in the lens section.
http://www.hasselbladdigitalforum.com/index.php/board,20.0.htmlFeel free to give me a ring at the studio should you have further questions.
Cheers
Nick-T