Merc and MSD ignition are both excellent choices with blower application. From my experience, MSD give you the advantage of multiple sparks when you turn low rpm with no boost and low static compression ratio
from a blower prepared motor. But the best thing of this system is that you can use their timing retard devices (according to rpm or manifold boost pressure) to lower the ignition timing when you are on boost. Remember that boost means less timing. More you boost, less timing you should have. If you put a total timing of 24-30 timing with any regular ignition system, your initial timing will be too low and in the non boost situation, when you need good timing to burn the poorly compressed air/fuel mixture, you will throw away unburned gas. Meaning pollution and gas money from your wallet.