When choosing a deco gas there are a couple of diferent angles to look at. WL's choice of a single tank of 50% is a popular one and it minimises the kit and simplifies the dive with no possabuility of a wrong gas switch and virtualy no possabuility of a CNS hit.
The two deco tank option on dives to 55m is laughed at by some on the grounds of what do you need all that for? However apart form potential bail out, it extends your bottom time by alowing longer deco hangs within the gas limits of your tanks and or cuts down on deco time in the water on shorter dives.
When choosing the gas the options include, potential deep bail out, prefered gas switch depths, CNS loading, deco curve, O2 window, deco on the fly or multiple slates, air breaks or no air breaks etc.
As for the 80% 100% argument its a non argument. 100% is the best gas for deco. However it can cause problems with its need for very shalow deployment and CNS loadings so some divers choose to compromise on the optimum off gassing gas and go for a reduced PP02 (reduced CNS) and a more flexable depth option for the shalow stops.