Under the hood a Tween is a Timeline, as in they use the same code. The Tween behaviour was a nice by-product of developing the Timeline feature. At the time of it's development, a lot of users had been requesting a built in Tween behaviour for Construct for a while, the Timeline feature was not quite ready yet, but it was good enough to re-purpose it's internals, and thus the Tween Behaviour came to be :P
If you think about it, there is only one thing which is unique to Tweens, and that is the Value Tween. You could do something similar by animating an instance variable in a Timeline, it's just not as obvious. Maybe in the future Timelines will support using a value property track, it is not high priority though.
To sum it up, Tweens are limited Timelines, anything you can do with a Tween can be done with a Timeline. They offer less functionality, but in exchange are easier to use. Sometimes you just want to get something done quickly, and a tween will get the job done most of the time. If you find that tween's are not quite making the cut though, then you might want to try Timelines.