I'm not gonna review the other ones because they're crappier. I am, however, going to say that making upgrades like this is meh. If you look at the Blizzard techtree upgrades, you'll notice that it isn't just a simple icon that gets new features with every upgrade, it changes color, shape and everything, but it still stays somewhat the same.
Now, as for the icon itself, I find the top of the shield to be kinda messy. The horns could look sharper, the shading is a little messed up as well (top right corner is the light source I assume, you might want to make the background brighter up there as well).
I would like to see another version where you zoom in a little closer to the shield (not TOO much, but a little).
3.5/5 for now.