Its very much a game, every scenario has specific goals with a time limit and be prepared to get a game over tbe first few times because of unpaid taxes. It's actually a pretty difficult game for the genre, and really requires you to plan things ahead (Buy land ahead of time to avoid nasty expenses later on!) I also found that the scenarios really work well for making you move out of your comfort zone and use approaches you normally wouldn't in order to reach the goals.

Edit: Also, there isn't really a sandbox mode in the traditional sense. You can make your own maps/scenarios and you can enable a "Free mode" for a scenario, which gives you infinite money (so you can't lose anymore). I never used the editor, so Im not sure how free you are in making a scenario with a practically infinte time limit to get something similar to traditional sandbox mode.