I've been wondering about this myself actually, "umm"ing and "ah"ing over which is better - MoT with shields or GWs. (As this is for a Tzeentch-themed army - my first for 8th - I've only been looking seriously at those two.)
It seems to boil down to a matter of horses for courses, really, but if pressed I'd give the edge to the GWs. Now that you always get to hit back unless you're more or less wiped out, striking last doesn't hurt as much - though it does make champions utterly worthless. Math suggests that GW marauders will do better in combat, as the extra casualties GWs inflict more than make up for the ones that shields will absorb. This seems to hold true for everything from night goblins to Chosen.
Having said that, if you're after a tarpit you probably want shields. Parry might save you an extra 1-2 marauders per turn, which means you're slower to lose ranks, which means you're sticking around longer. If the general and BSB are in range, odds are pretty good that anything hitting a block of 50 MoT marauders with shields won't come back out any time soon. (As long as that "anything" isn't also 10 ranks deep, of course.)
Why haven't I looked at flails? Worst of both worlds, mate. Avoid them like the plague in any but the fringiest of circumstances.
Four Colditz salads and a Hermann Goering.