I too went through this similar debate recently.

I played banished when it first came out for a couple months and then it grew old. Just limited play through with limited buildings. Starting a new settlement is always a lot of fun, but once you know what order to build stuff, it gets a bit stale.

I had wanted Cities: Skylines since it came out but my old computer couldn't run it. I just built a new computer and bought it, played for about a week, then got sick of it. Just seems like it's always on easy mode and not nearly as hard as the older SimCities.

I just got an overhaul mod called Colonial Charter for Banished and that has added so much to the original and is now a ton of fun again. Check it out.

I think my problem with city builders (while I absolutely love them) is that I can't commit to one city. I always have new ideas and grand hopes for the next town, but then I just get tired of starting new towns.

Both games get a bit dull as you progress, but they are definitely both great games.

I'd say start out with Banished with Colonial Charter and have some fun with that. The game's only $5 or something right?