Against a rushing attacker who stays fairly upright, I actually prefer the double leg takedown. An untrained rushing opponent will have tremendous forward momentum, and by taking his feet out from under him, you can usually put him to the ground really hard. If you pull his legs to the side, you can often throw him hard and get to his back for a choke.
As far as a theory for BJJ fighting goes, I would suggest this: control your opponent's movement first. This can be with a gaurd, full mount, side mount or cross mount. Once you have your opponent's movement controlled, the number of options you have (grappling or striking) is huge.
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