It's a tip-off (for me) as to which one is more likely to have a grooved receiver. If you should ever want to add a scope to it, that's a bigger deal.
6 port = Might have a grooved receiver (early model)
2 port = Probably won't have a grooved receiver (late model)
It isn't a "written in stone" rule. These rifles were mix-mastered in the post war refurbishment program, so you never really know for sure.
6 port is louder in my opinion, I have both and if I let someone shoot my 6 port svt then I hear louder flop sound of the muzzle brake. Not so with 2 port one.