Rules to identify the most stable resonance structures
You will encounter situations such as these when the two rules seem to contradict each other, or they confuse you.
However, when this happens, you must use another criteria to determine the best resonance structure. Specifically, you must take electronegativity into account. 

What this means is that the most electronegative element should have the lowest formal charge, and the most electropositive element should have the highest formal charge.
This is because the electronegativity "rules" explained above are what are likely to happen (that is how thiocyanate is likely to appear). This makes sense because the higher EN and element has, the more likely it is to "hog" the electrons and have a lower formal charge.
When you use these rules to evaluate the resonance structures of the $\ce{CNS^{-}}$ ion, you can figure out why the first resonance structure is preferable.