The 16VT will be a better swap. It can be made as fast if not faster than the 20V, it is also smaller, and weighs less than the 20VT.
The 20VT with gearbox weighs about the same as the Alfa V6 with gearbox.
The 16VT will be about 40kg lighter. Its still heavier than the TS though.
The only problem is 16VT engines in good order are rare, and things like cranks are expensive as they cannot be reground.
You will also need to upgrade the springs due to the extra weight. I suppose the Fiat coupe items could be used though. The fiat also has thicker anti roll bars than the QV.