Is the initial sneak attack more powerful with hitting both buttons at the same time for the crossbar stroke, or doing the flurry?

The flurry is a power attack. Power attacks tend to be more powerful.

Does the first hit from sneaking do more damage than the crossbar?

Power attacks get sneak attack bonus to further boost their damage.

So the best way to get the nifty kill animations from the trailers is too snaek up behind and do a power attack with dual daggers?

No, you just power attack for huge damage. To do the stealth kill animations is to normal hit while being very close and undetected, but only for man or mer.

A bunch of us have actually tested this.

Highest damage we think is double dagger attack from sneak, with the appropriate perks of course.

However, you'll almost never get a kill animation from doing this. If ever.

You mainly get kill animations from 1h regular and 1h power attacks. If you really want to make a kill animation every time, I recommend a single super awesome dagger in your right hand and do 1h power attacks.

How are you testing this exactly? Even with the correct perks and enchants, daggers are the lowest damage of any one-hand weapon, even when sneak attacking.