pjz99 is right, uv mapping a sperical shape is always going to be a tradeoff, the whole purpose of uv mapping is to take a 3D shape and turn it into a flat surface so that a flat image can be applied to it, think about trying to flatten out a basketball without stretching out the surface, the only way you can do it is to slice it into a lot of pieces. so what you end up with is a map that's hard to deal with because you end up with seams between the split pieces. or you can try to map it as one continuous piece which is what spherical mapping attempts to do but than you are effectively squashing the surface flat, which means stretching, resulting in distortion.

In this case if it were my model I think I would split the whole helmet map down the middle with each side being a separate piece, than you can flat map them and stretch them out along the edge where the seam is somewhat to help minimize the stretching and that ridge down the middle would hide the seam for the most part.