Crazy idea - I love it.
As others have said, going with a mixture of the two layouts would probably work best, representing major arterial roads through the city with smaller sections of more densely populated areas. I would definitely think you should pop in a few 'parks' and squares - which will also reduce the amount of detailed building work.
On the terrain pieces, roads that are 150mm wide ought to be 'boulevards' with a small patch of grass and shrubs or even small trees in the middle. The 75mm wide roads would still have pavements etc. and you ought to consider painting minor roads and passageways inside the building bases. For intersections, I suspect you will need pieces that are 150mm square, 75mm square amd also 150mm x 75mm. And these road sections ought to represent corners as well as junctions and straight sections.
So if you want them to look correct when in-situ, you may actually need to make a whole lot of these 'road' pieces. As a consequence, I would suggest thinking hard about the roads rather than just about the buildings as ultimately I suspect that this may be a lot more work than it seems . . . . .
