whats the difference between these 3 intake mainfolds ????

 

From the various tests I've seen on different motors in magazines and on the net by people like Duttweiler, Livernois etc. the best is the LS6 which makes approx. 15 more Hp on a cammed motor than the LS1. The LS6 has a larger plenum and runners and the bottom is flat and sits right down on the valley cover, which necessitates clearancing or changing the coolant lines on an LS1 engine for it to fit properly.
The LS2 manifold, although it has the larger 90mm Throttle Body, makes the least power on every test I've seen, even with a stock cam in an 6.0 truck motor (LQ4). The best cathedral-port manifold seems to be the FAST with the 90mm (now 92mm) TB. The LS6 will give you the best bang for the buck IMHO, especially if you can find a used one.