The Glamorgan trainer intends to give both the chance to prove themselves, for owners William and Angela Rucker, in this weekend's famous handicap - for which a final field, up to a maximum of 24, will be declared on Thursday morning.
But Williams admits the complicating factor is likely ground conditions - currently described as good to soft, with a dry week forecast until a possible return of rain on raceday.
"I'll be brutal - one needs good ground, and one needs softer ground," he said.
"So I'm in that tricky situation - the ground is drying like hell, but they do give a potential downpour on Saturday, don't they?
"So it looks like it could be the right thing to leave them both in there and have a crack."
Quoi De Neuf, last seen when well-supported but unluckily brought down behind Not So Sleepy in another showpiece Grade Three handicap at Ascot just before Christmas, could do with the sun and drying wind continuing.
Mack The Man's hat-trick bid will, by contrast, be aided if wet weather returns in time.
Williams added: "If we get rain it could suit one and if we don't, it could suit the other.
"The ground will be beautiful there - but if it's drying, it will definitely favour Quoi De Neuf, and if we got that rain on Saturday then it would definitely play heavily to the strengths of Mack The Man."