I have and use both KONTAKT and an XV-3080
The XV's sounds are not bad, but I would never use them "dry" on a track... The main strength of the XV is that you can browse through a thousand instruments within a few minutes, not that the stuff soudns that realistic. I am sure the extension cards sound great, but I dont have any.
For stuff that needs to be more realistic, such as solo instruments, I load in instruments from CD-ROMs into KONTAKT.
KONTAKT by itself is not "realistic" sounding, it's just that you can load in new stuff... if you load in a bad soundfont, it will sound bad... if you load in stuff from $$$ cds such as Vienna Symphonic Library (VSL), you will get AMAZING instruments, etc.
What I dont like about software samplers is that I often dont like to wait for the computer to boot up, route the necessary tracks to the outputs, etc.. basically set it up... I want to play RIGHT AWAY.
If you dont care about clicking around the screen, by all means get yourslef the sweet KONTAKT and buy a few very nice libraries (VSL has a new "best of" CD-ROM at around $700).
Otherwise if you want instant results, get yourself a hardware sampler with a hard-disk (external or internal) and plenty of RAM, and dump instruments from a few CD-ROMs into the sampler's drive, and go to town... Just push that power button and you'll have YOUR library with the best sounding stuff you could find/buy.
I hope this helps.