I've worked with Promotion some, but haven't really gotten to know it well enough to judge it on features. But that fact alone gives Graphics Gale a big boost in my opinion. There was basically no learning process needed, beyond discovering what shift and ctrl do in the palette editor.
The main gripe I have with Gale is the limited number of brush sizes. It would be much better if you could just set the diameter in pixels, ala oekaki. Maybe select shape too, but I rarely if ever find myself wanting to plot giant squares anyway. Also, oekaki-style mask/remask would be nice.
What I love most are the drag-and-drop palette editor (not perfect, but better than anything else I've tried), easy setting of different grid dimensions, easy working with layers and frames.
Gil, what do you mean by pixel snapping? I thought you just meant that when zoomed in, it shows you where the pixel will be drawn if you click. But I just opened PM and it does that too.
Oh, and another thing that would be nice is automatic conversion between frames and sprite strips. Although I suppose special-case tool like that are what lead to a cluttered interface like PM in the first place... Actually it looks like PM does have frames to strip at least. I really should spend more time with it. It probably is the better tool, but GG has an amazing balance of power and simplicity that keeps the battle going.