a) Micro ability: Low; More marauders, High; More Marines
b) Game time: Early; More marauders, Late; More Marines
a) Marines will provide greater DPS/better engagements than marauders in a lot of situations. If you can split, kite, and flank well with them, they will be better than marauders. You will also be saving gas which can be put into more ghosts and/or vikings, which again with good micro will benefit you more
b) Early on when you're vs mainly gateway units, marauders /w conc shells are vital. As the game progresses, his zealots will have charge, his stalkers will have blink, and factoring in the gas as mentioned before, marines become a better option overall
But unless there are exceptional circumstances never go 100% marines or 100% marauders. Even if you have 'perfect' micro, marauders mixed in or leading the front will of course tank some key damage, or allow you to take advantage of the mistakes of the toss and snipe a colossi extremely quickly or pick off retreating units etc
EDIT: Masters toss btw