From the sound of it, an uncapped Cities: Skylines region is equivalent to a good chunk of a Simcity 4 region, except everything is connected. This city size may not be officially supported, but it's been said to be an option, and I certainly think that a real interconnected simulation is worth trading a few square kilometers. Simcity 4's model is a measure that really only made sense in an era where much larger maps were desirable but not technically feasible due to the simulation.
Keep in mind that Simcity 4 is still eminently playable. By the time Skylines is that old, everyone will be able to play at the maximum map size, and the map may well have been made larger via an expansion.
Keep in mind that Simcity 4 is still eminently playable. By the time Skylines is that old, everyone will be able to play at the maximum map size, and the map may well have been made larger via an expansion.