Population limit:
Cities: Skylines: 1 million
Simcity 4: Unlimited

Max playable area size:
Cities: Skylines: 9.6x9.6km
Simcity 4: Technically unlimited, although very large maps take a lot of effort to make. (Biggest maps currently downloadable as mods include the entirety of Luxembourg on a 1:1 scale, and also the main Hawaiian island on a real life scale as well.)

Wealth Classes:
Cities: Skylines: there are no wealth classes, every citizen has the same wealth and the same types of houses get built in every area of the city.
Simcity 4: People are of different wealth, gentrification and urban blight are a thing, and you won't see mansions and middle class villas in polluted areas of the city with bad education access etc.

Skewed population numbers:
Cities: Skylines: Have you ever noticed how there are ~20 people living in a skyscraper? Well now you do.
Simcity 4: Skyscrapers provide jobs to thousands of people and put a heavy toll on the roads around them as it draws commuters towards it.

Price:
Cities: Skylines: $320 with all dlc
Simcity 4: $20 with all expansions included (often goes on sale for $5)