Henrik,
As mentioned above, the multishot backs outperform the scanning backs. I use a Sinar 54H. The 4-shot results are outstanding with rich colors and no artifacting. When extreme detail or large prints are needed, the 16-shot capability is there. Taking 4-shot images is basically foolproof. On the other hand, the 16-shot mode requires much more careful technique and equipment. In 16-shot mode, the sensor is displaced by a half-pixel width (4.5 microns) in each direction to increase the resolution by a factor of 4 (2x in both horizontal and vertical dimensions). You must make sure that your sensor is calibrated and checked frequently (easy manual process), but most of all you must have no vibration in your studio. Even someone walking nearby in their bare feet during the 16-shot exposure can cause enough vibration to blur out the finest details.
Here is the basic math. The 4-shot mode with the Sinar 54H (9 micron pixels) has a maximum resolving power of about 55 lp/mm. With the 75H (7.2 micron pixels), it would be about 69 lp/mm. These are very decent resolving capabilities, and there are a variety of lenses that can reach these abilities into the corners. However, with 16-shot mode of the 54H, the maximum resolving power is now doubled to about 110 lp/mm. So, not only is careful technique and the control of vibration absolutely critical, but the choice of lenses is much narrowed in order to reach this kind of resolving power into the corners.
So, 16-shot backs like the Sinar 54H offer the highest resolution for fixed position camera shooting (ie, non-stitching), but can be a PITA to get it right. The market for those who are willing to do 16-shot commercially was too small, so these backs were suspended from being manufactured. I use the term "suspended" as opposed to "discontinued", since one of the engineers at Jenoptik recently told me that they would gladly produce more 54H backs if the demand was there.
David