Although my Kontakt demo has now expired and Mach Five doesn't currently have a demo version available, I am leaning towards Mach Five based on my overall experience with MOTU software vs. Native Instruments software.
The GUI just seems to allow for a more intuitive and flexible work flow. The drawback, asides from release date slippages, is the lack of native libraries, but as long as translations work well, that shouldn't be a major problem.
And as far as translations go, the newest problem seems to be that most samplers are turning into software ROMplers and thus have less commonality than before, making it harder to do 1:1 translations than with older libraries.
But this would be a problem for any of the major samplers. And I guess that then starts to push people towards the samplers with the perceived "best" libraries, taking us back to the early days of hardware sampling

.
In some ways, standalone VI's supplant the need for software samplers for many of us, unless we see ourselves doing major sound design. In my case, I save that time for creating fresh sounds vs. fine-tuning natural or otherwise recorded sounds. But in the area of background sounds for movies, there is an advantage to manipulating "natural" sounds vs. synthesized sounds, so a software sampler is still on the agenda.
At any rate, I didn't like ANY of the Native Instruments demos that I tried (and I downloaded every demo available). I understand that FM7 has a new version coming out soon (presumably a better GUI), and certainly the current Kontakt is an improvement on the previous, but in general I find that MOTU GUI's seem to be informed more by how musicians actually work (putting aside the infamous incorrect instrument ranges of MSI).