I'm wondering about the same thing. From the quality of the online demos I've heard, I would say that it sounds better than the rest. The whole explanation of the sample trigger method (the Ram preloading of the initial sound of each sample) seems to make sense. Technically speaking, I would imagine that the Gigastudio sounds better than the rest. If you've got a powerful enough computer, Gigastudio's going to really put it through the ropes.
I don't think that samplers sound any different at all. In my mind there are two things that could make GS sound better:
1) Gigapulse, if you like the sound of the IRs.
2) More established platform, therefore many of the composers using it are also more experienced at programming and/or mixing.
I currently have three GigaStudio PCs (two are being phased out ATM) and have had no problems with them. They have been rock solid in spite of having been thrashed regularly! I also have Kontakt 2, which has been the most awful program that I've ever had the mis-fortune to use. However, there are many people who will say the opposite:>)
My advice would be that if you intend to use GS on its own PC then it may well be the best platform for you. However, if you want to run everything on one PC, then Kontakt might work better. You also will have access to the K2 "scripts", which might be of use to you. I think that you can do the same sort of thing in Giga, but am not sure that it is so user friendly.
You will see that some people on this forum have terrible trouble with Giga, but do remember that this is the Giga forum. Just go and have a look at the NI forum (not the NI section on this forum) before you think that K2 is all plain sailing!
I have been blessed with GS3 Orchestra. I'm having trouble w/ some Akai conversion, but the quality of samples w/ Gigapulse and Blue Tubes VST FX is impressive. I run the app. through my Creamware DSP cards and couldn't be happier. As a live performer stability is paramount. This combo toured for 6 months overseas and USA w/o incident.