It isn't an internal/external thing, but more a stock vs sfi issue. The balance is achieved by opening up some half moon slots to match the opposing full holes, removing some pressed in weights and then balancing.
For competition use the PS book recommends heli-arcing the started ring to the plate and drilling new smaller crankshaft mounting holes that are a tight fit for the bolts. The latter couldn't be done in an external balance (assumption alert!), so one might conclude that the stocker would be better off in an internal balance scenario.
If the price is right I'll probably go with the rebalance part only since I'm building a slowazz 11.5 second car with a brick outhouse shortblock. Then I can step up once I learn how to drive and tune (or should that be tune and drive?).
HTH,
Tom