GGKK,
Certainly no expert, but it would appear based on what I've read and heard, that the main difference is that BFS twitches tend to be global, harder, and move throughout the body. ALS related twitching tends to be more fine and localized initially.
However, that's not a hard fast rule (you will hear folk on both sides with different experiences), which I think is why you will hear time and time again, twitching is not indicative of one thing over another. Muscle twitching in and of itself is a specific as saying someone is running a fever. Yup something is up, but 99 times in a hundred (acutally more like 9,999 times in 10,000 or more) it's an issue that is uncomfortable or annoying, but not life threatening.
When I mentioned muscle twitching to the neuromuscular disease specialist at the U of U, he did the politest equivalent of an eye-roll and said, "That's not very specific is it..."
Take care,
Robert