How much of a difference would brand A pistons that weigh 420grams each vs brand B pistons that weight 390 each.

Is that a significant difference?

 

It depends on how high your spinning it. Sometimes a lighter piston will keep you from having to add heavy metal to balance crank. Most important is to make sure combination of piston, rod, and crank are compatible for what you are trying to accomplish. There is nothing wrong with a 420 gram piston.