If I had a very clean (or vector) version of the map on the right I'd bring it into photoshop, separate the streets, buildings, etc. into layers (try select color range to create a selection that you can fill on a new layer) and then work on those layers separately. Then you can paint on textures, add additional detail separately.

The one on the right looks very cool...but it doesn't look much like a city. I will say that Ravells' suggestion would be a cool way to go...if you had the cash. I've seen some neat sci-fi city stuff turned out of that.
M