Whats the difference between internal and external balancers? How do you know which one to use?

Chevy 400's and 454's use the external balance from the factory.
Some 383 stroker kit's need them too.
What engine are you talking about ?

350

350
=
It's internal :)
An external one has weight added or removed from one side so it acts like added counterweight to the crank.

O.K. THX for the help.