Putting some theory work on it...
Rune:
3 (Draw*) -2 (Discard [random]) -1 (Activation Cost [3]) +3 (Multiple Activations**) -2 (Unstackable***) = 1
Sigil:
3 (Draw*) -2 (Discard [random]) +1 (Activation Cost [1]) +3 (Multiple Activations**) -2 (Unstackable***) = 3
*Based on observations of Precognition
**Based on observations of Rustler.
***Determined due to the multiple use, which makes copies useless (similar to Nightfall).
Just a design question, but does the draw still work if you have no cards to discard?