Internal V's external balance

[effie351]

G'day Guys
I'll display my ignorance here.
What is the differance between internal and external balance?
Is one better than the other?
Scat lists their stroker cranks with 6.125" or longer rods as internal balance and 6.0" rods as external balance.
As I havn't bought pistons or rods yet and will be balancing the bottom end, which would be better?

Thanks Mike

Ps motor is a 393 Cleveland