I used to always use Wonder Under, because I liked the lightness of it, but have had some problems with it. I used it on my most recent quilt. I find it sometimes allows the applique to come off before I can get it stitched down especially really small or narrow pieces, and doesn't protect the edges from fraying as well as Steam a Seam 2. I find Steam a Seam 2 makes is just a little thicker and sometimes that is a negative, but I still think I would choose it for raw edge applique now.
I haven't tried Steam a Seam light--I'll have to give that a test. I like to use the narrowest of edge stitches, usually zig zag, and sometimes, if I use Wonder Under, it sometimes frays in spots when I wash the quilt after completing it, necessitating additional stitches, which makes the back less appealing and makes a more noticeable edge.
I use Elmer's washable glue, which I heat set, for turned edge applique to baste it until I stitch it.