Why not both? Without Joe Guardsman the marine becomes the guardsmen as bigger better marines then just chop down waves of tacticals.

Why not set the stage by outliing the 450km front, and then tell us how a drop pod assault of just x number of marines at the crucial junction at the crucial time will do what Joe guardsmen cannot do, swing the battle. Use their stormbirds and thunderhawks to insert tanks and armor at places no sane human would even try. Teleport dreads into command bunkers after ninja marines get the teleport hommer set up, etc etc. But to a backdrop of actual warfare.

As i said earlier for me the best stories are the ones where the marines act in a setting of imperial warfare, Iron within has gene ogryns and Imperial army dying in droves so their iron warriors master can do what must be done. The serpent within has base security maned by humans so the Alpha legion can out alpha legion eachother without needing to be manning guard points. Master if Mankind has the Mechanicum deploy LEGIONS of tech thralls and automata so the CUSTODIANS can do their thing. etc etc.