Hipster: The unbuild of the ROM's heritage wings ( removing all those dropped ceilings and blocked windows ) was, I think, a bigger job than the AGO faced with sprucing up their heritage galleries around Walker Court. But the ROM Rotunda was left as it was, mosaic ceiling and all, whereas the Walker Court was extensively remodelled, so maybe it's a draw.
I think the "new build" comparison comes down to more than a Crystal vs. Titanium Box cage match. The bones of the Barton Myers building obviously play an important part in Gehry's addition, whereas the ROM's heritage buildings weren't changed structurally. But the Terrace Galleries were entirely removed. I don't know how you'd decide which wins. The workmanship in the Stair of Wonders suggests that the AGO might, if you base "build quality" purely on what shows on the surface. But, as we know, the Crystal required an entirely new set of construction methods that the AGO didn't.
Both institutions installed white oak floors, quite nicely I think.
The AGO installed sturdy and rather ostentatious Maple baseboards, whereas the Crystal has the much-talked-about drywall that kiddies love to scuff up.
The ROM has considerably more custom-designed display cases, because so much of their collection is three dimensional and displayed away from walls, but the ship model cases at the AGO look just as well made to me, as do those housing the Frum Collection.
The ROM's German-made cladding isn't tonally uniform, but then neither is the AGO's Japanese-made titanium.
Where does it all end?