Skill Mod % is for non maxed, preferably non sib weapons. It can boost your HA with the weapon, though it does not visually change your HA on the weapon. This can be analysed using a tool. These have been documented as Doer has said. For a non sib setup you would want the best mod % boost for least amount of peds spent. This should be Terrus sights and seizzt ranger scopes.
Skill Bonus % is for any weaponry non sib & sib. It can potentially increase gaining additional skills when attached. The best skill bonus attachment that I've seen for ped spent,has to be Headshot 1 + Seizzt Laser Sight 2000L & Seizzt Laser Sight 2000L+. Gives a 1.8 skill bonus boost at a cost of 0.06 = 30 factor. Nothing else I've analysed gives this high a factor boost. I personally use it on my gauss weapon.
