The most flexible approach is to use the graphics object of the
MovieClip class and paint your lines dynamically. Then (as you suggested), apply a Glow effect to this display object.
With the graphics object, you can paint many lines in a single
MovieClip), so it's a good idea to do that and have the filter applied to the Sprite, so that all lines will get the same filter effect. If you're drawing lines over a huge area, you should know that the glow filter is limited to certain size. From the docs:
A filter is not applied if the resulting image exceeds the maximum dimensions. In AIR 1.5 and Flash Player 10, the maximum is 8,191 pixels in width or height, and the total number of pixels cannot exceed 16,777,215 pixels. (So, if an image is 8,191 pixels wide, it can only be 2,048 pixels high.) In Flash Player 9 and earlier and AIR 1.1 and earlier, the limitation is 2,880 pixels in height and 2,880 pixels in width. For example, if you zoom in on a large movie clip with a filter applied, the filter is turned off if the resulting image exceeds the maximum dimensions.
I don't know how your background looks like, but with a dark background and bright lines, I suggest you additionally set the
blendMode of your Sprite to
SCREEN (see BlendMode). With dark lines on a bright background,
MULTIPLY could be a good blend-mode.
Update: In terms of performance, the fastest approach is probably a simple image of a blurred line. But that's not very flexible, as you would need an image for every possible line-length. A vector-based approach (with gradients or different fills) is going to be slower than the glow-filter. These filters are very fast and optimized and will outperform the drawing of lots of complex vector shapes.