My friends in the urban economics biz pay most attention to the residential sector. It is surely big, but there is a lot more to cities than housing. Here is a link to some New York research being done on urban retail by my USC colleague Jenny Schuetz and her co-athors.
I have tried to to find national shopping data by location. This is complicated. The International Council of Shopping Centers reports that there are about 100,000 “shopping centers” (including many “Lifestyle Centers”) which account for about 50 percent of all U.S. retail gross leasable area. Whereas some of the centers are Edge Cities which include many of the functions once associated with downtowns, many others are developed at low densities that are seemingly under the radar (as in Edgeless Cities).
So, rather, than downtown and the ‘burbs, it is an extremely complex arrangement. And that's not bad. From NHTS data, it appears that the many small “centers” and other retail places are spatially distributed so that the average shopping trip by central city vs. suburban residents in the U.S. were of similar duration, 14 minutes for central city residents and 16 minutes for residents of suburbs. In fact, the 2001 and 2009 travel surveys show that the average shopping trip distance in 2001 and 2009 was the same at 6.7 miles each way (not walkable, but not bad).
Note that in the intervening years, U.S. population had grown by 7.8 percent but shopping access had not gotten out of reach in spite of significant growth and development. Not bad.