I've sat under various types of preaching. The topical, sometimes referred to as Historic Redemptive, of the Dutch churches are strictly within the topics outlined in the Catechism. The traditional Dutch church will have one Catechism sermon per Sunday (hence the division of the Heidelberg Catechism into fifty-two Lord's Days), and one topical, usually in the morning service. This morning sermon is usually topical in the sense that it most often ministers to the particular need of the congregation, allowing the minister to do the job he feels he came to do in the congregation.
So this topical is never slanted by being narrowed without the context of a throrough theology, since that is covered in the afternoon sermons.
I've also sat under the expository sermons. Mostly that has been a sad affair for me. But nonetheless, I've also seen the benefits of that kind of approach. What was practiced in my church was a ten to fifteen minute lesson from an elder from a regulated exposition from the Confession or Larger Catechism. And then later the minister would give a sermon on the series he was taking up at that time. One gets to know the books of the Bible that way. And if you keep notes then you can always compare, and take down the many cross references that always come out.
If you're a Bible marker (I absolutely refuse to do that) then expository is what you would like, I think. But if you're a relational thinker, like I am, then topical is the one. But both can be beneficial. I don't think that one excludes the other, or that there has to be a choice between them. I think that a minister can do both, even with a Catechism sermon in there.
I've seen Bible markers in both scenarios. In the topical approach, interestingly enough, they tend to mark larger sections of the Bible; the with the expository approach individual verses get marked different colours. But I suppose that differs from person to person too. All the same, I won't do that, ever. Well, except once, with an NIV, which I didn't use as my regular Bible. (In thirty some years of going to church as an adult I have only had two Bibles I took along. )
For me the differences in the approaches has not been what type of preaching was practiced, but the faithful attendance to doctrine. I did not see any advantage to one or the other in that regard. But I suppose that it can be described as the difference between Biblical Theology, or doctrine, and Systematic Theology, or doctrine. But I don't think you can have one without the other.