Having played the SimCity franchise religiously since SimCity 2000 and up to the newest SimCity released a couple of years ago, I can honestly say that Cities Skylines brought me a lot more enjoyment in terms of game-play, challenge, and logistical problem solving. I will preface this with the note that I am a bit biased towards Cities Skylines as I liked the SimCity 2000/Simcity4 style of play, but that in mind, I think Cities Skylines gives a more involved and realistic simulation experience for the reasons included in the list below:
1) Cities Skylines allows you to buy land. I know this sounds like a menial thing, but many of the community's gripe with the newest SimCity was that you were confined to a small area. It seems like a lot of room at first, but you soon realize that your ability to diversify the builds of your cities and styles are severely limited by the grid if you want to maximize your space. Cities Skylines allows you to make some HUGE cities and that brings a lot to a simulation-style game, especially with the development of processors that can handle more data and calculations than ever.
2) Cities Skylines introduces pipes that you lay down and aren't automatically connected with the roads. I know this may seem like a little thing as well, but I think that as a city builder, the layout of your city should be determined by the architect and not a predetermined algorithm. The ability to customize the infrastructure of your city to a minute detail in Cities Skylines gave it a much more satisfying and enjoyable experience when things went well, and more of an ability to problem solve my design when they went wrong.
3) Community support/steam workshop. Cities Skylines makes this a HUGE part of their game, and it really shows with the modability of the title. SimCity seemed a lot more pre-manufactured and static because you were left to buy expansion packs and DLC where as the steam workshop and community provided some really cool buildings, maps, and the like for free.
4) Graphics. I know, superficial, but Cities Skylines is BEAUTIFUL. I'll just leave it at that.
I hope this helps a bit, definitely read/watch other reviews but I preferred Cities Skylines over the newest SimCity because it brought a very rich experience to city simulation that I had been looking for since SimCity 4.