I voted for the SIGGRAPH video because:
1) Its cheaper for me.
2) There's nothing quite like watching other people use A:M
3) It helps to know the basics before moving on to the advanced
In my opinion A:M Paint is the better deal but it might not be the best choice.
Only you can determine that.
If you learn and use A:M paint you'll be using it well past the shelf life of the SIGGRAPH DVD.
Once most people watch the DVDs a few times they usually put it on the shelf.
Decisions... decisions...