what's the diff between the 5A and 6A? advantages, which is better, increases in power /gas mileage

 

The MSD 5 is a lower output type box for daily driver use with no increase in fuel load, I.E. Cams, manifolds, carbs, etc.
The MSD 6 is designed for as said above, an increase in fuel load, and or compression with a greater power output to overcome larger static loads. Keep in mind you'll need a good coil to make it all come together. In switching to a Capacitive Discharge (CD) ignition, the coil will get a full saturation (unlike conventional ignition) and will overheat more rapidly. Spare no money on the coil or it'll leave you stuck one day.
Hope that helps.
Chris