City vs. Suburb is a bad dichotomy

City/suburb isn't actually a very good dichotomy, because we don't have good definitions for either. I prefer pre-and post automotive urbanism, and most places contain a mix of both--in some jurisdictions, it's 100% of the land area, and in others, it's .001%. What determines that percent is what percent of the city was built before the advent of mechanical transport.

It's easy to forget, but deep into the 19th century, horses were a regular part of urban life. If you were rich, you had a coach. If you were just affluent, you had a couple of horses. For those without, you might rent one from the livery stable, or you might rely on grabbing a ride on a horse-drawn wagon.

It's weird to thing about, but most of our urban transport simply replaced horse-power with mechanical power. The switch was abrupt, and not just for technological reasons, but for biological ones: The Great EpiZooetic killed a lot of horses, and horse-power became more expensive. 

Classic urban model of land values is the 'teepee' of maximal value in the center, and an arcing decline from there. But that's a simplifying model, and doesn't reflect the complexity of actual conditions. There also exists a class of what I'll call 'carriage suburbs', which are affluent single-family detached semi-mansions located at the extreme edge (>.5 mile) of a pre-industrial 'walking' city. Prior to the advent of mechanical transport, there was never a pure 'walking city'. There were always suburbs and exurbs made accessible by horse or carriage. You see it in Rome, you see it in small towns, you see it in big cities. Model also fails to reflect height: all else equal, elevation is always a good thing-less flooding, less smoke, better views. In combination, the result is that the 'heights' of any city get occupied by an affluent class with access to non-walk based transportation. 

Past that, it's just a matter of density. The more limited the supply of land, the higher the resulting density. Barcelona being the extreme example case--rigorous denial of construction outside the city walls resulted in very high interior density. Most other pre-industrial cities tended to barnacle on faux-burgs just outside the city gates--locations that were 'on the way', but not bound by city laws or nuisance restrictions. And once a fauxburg became dense enough, it was incorporated inside the city wall and became part of the city. And a new fauxburg emerged at the gates once more. 

Arguably, fauxburgs are a case of suburbs: I disagree--fauxburgs are much more shanty-towns, increasing in density and permanence. Essence of suburb is the commute--living outside the city and commuting to the city on a daily basis. (Exurb being characterized by a non-daily commute or an extreme commute*).

Railroad suburbs are an interesting case, because while they are technically suburbs, they are both pre-automotive and pre-zoning. Which makes them radically different from post-war suburbs in block size, land use mix, and density. Every commute still involves a walk portion, and the street network has a characteristic 'long block' pattern with many side-streets (NYC's Avenue/Street pattern) to maximize access to the rail lines. Secondary pattern being commercial sales near the station, and noxious uses (industrial/warehouse) directly abutting the rail line. Since they still 'made for walking', even travel is now multi-modal, railroad suburbs are still generally considered good urbanism.

Post-zoning, urbanism goes to hell, and there is rhetoric about the 'extinction of walking', and the assumption that everyone will drive everywhere, and walk no further than across a parking lot, and that there will be parking skyscrapers as-needed. So we see a street network at a scale suitable for the automobile, the omission of sidewalks, and the establishment of massive building setbacks to permit future road widening, and massive required parking mandates. And that's the legal superstructure on which (most) of America has urbanized under for the past 80 years. And it's been going on for so long, almost no one knows anything different, unless they go visit Europe.

* Call it 1.5 hours of travel time. Normal commute is half an hour, plus for minus 15m.