I think it's an earlier test and taking the nuchal measurement combined w/ blood test is more accurate. The quad test is just blood work and done later i believe at around 18-20 weeks. The N/T screen is done b/w 11 and 13 1/2 weeks or so. And I agree w/ Hannah (tankerchic) it's awesome to have such a detailed u/s that early. Mine was almost an hour (but i am preg. w/ twins so it might have been a little longer than normal).
It is my understanding that used in conjunction with the blood tests it gives the best results. And besided you get a really great u/s.