As far as I can tell, I get a certain amount of skills per PED cycled through the weapon. There is a large element of randomness in the timing of the skill checks, but over several days it becomes apparent that I get about 1 Rifle point per 50 PED ammo, regardless of which Carbine I use. Using a gun that delivers 10 dmg/sec takes longer than one that does 40, but at the end of the ammo, the skill gain is the same.
With active SIB, I also received more extra skills upon killing a creature than when the SIB runs out, so the rate was probably faster. However, the last time I was able to use a weapon on which I have active SIB was quite a while ago, so it is hard to compare.
The advantage of using a maxed weapon is having better eco with it. Using an active SIB weapon, i.e. not maxed, results in faster skilling, at the expense of some eco. What is better for you depends on what you are trying to achieve, and how much you are willing to spend on it.
