Simply stated, intensity is used to allocate memory for the scrypt buffer as a power of 2.  For intensity 13, 2 ^ 13 = 8192, intensity 20, 2 ^ 20 = 1048576.  So for each step up in intensity, the greater the jump to the next memory allocation setting on the exponential scale.  Xintensity in sgminer can be tuned on a much finer scale as it is not based upon powers of 2.  There are so many other variables contributing to optimal configuration that I will not mention them here, but there is plenty of information available out there if you really want to learn how it all works.  There are many nuances that vary by type of hardware, and simply increasing speed does not necessarily reap performance improvements.  And then you really need to evaluate if any performance improvements achieved are actually cost efficient.