just let you know, if you buy H2D you can't use film back. For this, you will have to buy the H2 + digital back. today you have 4 main brands ; Leaf, Hasselblad, Phase One and Sinar.
The sensor quality is almost the same for all beause there's only 2 different factories I think which are able to produce such sensors. The most important thing is the input soft and the soft you're using behind to treat your pictures like Flexcolor (Imacon), Leaf Capture 10 (Leaf), Capture One Pro (Phase One).