Cast/Forged Steel Crank, what's the differenct?

A cast crank is moulten steel poured into a form and then it gets machined into the finished crank.

A forged crank is basically the same except it's poured under pressure.

Forged is usaually a little more expensive and a little heavier. But is also usually stronger.

The extra strength is worth the cash and weight in most cases.

The forged is always going to be stronger, and more expensive than a cast crank. But which is better will depend on your plans. If you are using a stock block, a cast crank will likely out last the block ie the block will breal before the crank does, so there is no reason to spend the extra on the forged crank. A cast is probably good up to 400 rwhp, anything higher than that I would start looking at better blocks and a forged crank.
I don't have personal experience of blcoks or cranks breaking, but this is what I have been told by folks that know a lot more than I do.