From what I know of Roman, Medieval, Modern Europe, and US history...
Home ownership seems to have the best correlation to a middle class.
Rome:
Optimates and similar had huge estates and paid for the military (which is why they had control, and could make themselves emperor).
The Plebians & freemen rarely owned property. They performed subsistence farm work on the estates. Even lower was the slaves.
In the cities there was a small middle class which owned homes, ran business. Often the same building.
The poor rented apartments and ate fast food everyday (no kitchens in the apartments).
Medieval to Modern:
Middle class collapsed after Rome. But the 12th century mini-rebirth, and famous 16th century rebirth was marked by artisan and business middle classes exploding in the cities, again, buying homes. Especially in the cities where they were well equipped to protect property rights from the bullying of the nobles living in the country estates. See wikipedia French bourgeoisie - "The Modern French word bourgeois derived from the Old French burgeis (walled city), which derived from bourg (market town), from the Old Frankish burg (town)".
http://en.wikipedia.org/wiki/BourgeoisieUS History-
As an English colony the major lure of immigrating was not just religious freedom for the minority that is deeply committed. It was really the possibility of owning farms on the American frontier, compared to the expense of property in stagnant and crowded UK and Europe. The US was born in 1776 as a country were anyone could move to the frontier and establish a new farm and homestead. Otherwise, stay in the city and start a business/home. We were a country born almost exclusively middle class, which is very peculiar in history. I've heard a historian claim that we didn't have our first equivalent of a millionaire until decades after the war was won.
Home ownership is correlated to stable families, stable and important social/political connections, and productive lifestyles. These traits are not just western, but seem roughly universal across the world and across history.
http://www.theglobeandmail.com/news/world/article772689.eceSince homes are mostly a collection of basic commodities, mostly unskilled labor, and some skilled labor then it tracks very well with inflation (except 2006 bubble).
http://www.multpl.com/case-shiller-home-price-index-inflation-adjusted/