Have a look @ the results. They speak for themselves.
Whilst those turbos make the numbers at the top end, they are not as responsive onto boost, inbetween gears etc - and hence they are cheaper as their performance is not as good. So basically you end up with a mid-lower range that is not as good, slower car etc
Power delivery is generally not as linear either which results in wheelspin, slower car etc
Price wise also comes down to the parts they are made from. Bush/Journal bearing which is cheaper to manufacture than ball bearing turbos from people like Garrett/Billet/Precision etc.
If you want the best performance, and can afford the extra 600-800 for Garrett - do it. The turbo is the single most important part of any setup in terms of performance IMO as it's the thing that dictates how the car performs/responds and so on