There are lots of threads on SupraForums on this subject. There is a camp that says the QSV does give an improvement when compared against *I think* an open manifold with open hosuing, and there are graphs to prove. But there is also a camp that says a true divided setup will be just as good, but nobody has yet done a back to back comparison as it would involve a manifold swap (and most of the tests kept the same manifold and just swapped turbine housings and added the QSV - basically as it was SP running the tests on customers cars when they came in to get the QSV fitted)
I'm assuming you understand that the exhaust manifold needs to be open, or have enough of the divide machined away to work with a QSV?
Pretty sure a handfull of people stateside have used a QSV on turbos of 67mm size with positive results (ie the QSV is not purely intended for large turbos)
Both ways have added complexity, QSV obviously has the valve itself, the dual wastegate has obviously the plumbing and usage of 2 wastegates. I think at the end of the day you'll find it's 6 of one and half a dozen of the other.
Which exhaust manifold do you have (or are intending to buy), or is the answer to that question the reason for the thread?
If you want my sceptical view on this subject I think the decision of QSV vs dual WG is so insignificant in the overall final performance of the car you go with the option for which your current manifold dictates. I personally would take dual WG over QSV (mainly due to it being a more well trodden path), but I would take a single WG divided manifold over a dual purely for ease of fitting/less equipment/less weight on manifold