Both have their uses. With Flourite you'll need to dose the water column and use root tabs. With dirt you'll only need to dose the water column. However, rescaping with a dirted tank is a PITA. You'll also need to cap the dirt. Since you're going low tech, why don't you use some dirt capped with the fluorite?
Maybe it's just me but I have had bad luck using soil in tanks to any real extent. However, I love adding just a bit of potting soil 1/4 inch or so, to my low tech tanks. No need for it to be low tech, I just don't worry on my high tech tanks as much. Ironically, I have been using Flourite on my tanks where I add dirt, though on a low tech, gravel works just as well. I just like the look of Black Flourite sand better. I find it's a good medium point, you don't get the negatives of a dirt/MTS tank but still get some of the benefits.
A dirt tank would require a very thorough planning. One thing for sure, you shouldn't try to rescap your dirt tank once everything is setup. Dirt will flow up. A dirt tank would require longer period for the water column to become stable enough for fish. 3 weeks or more before adding fish. You get dead air bubble spot in the dirt and you need to poke the substrate once in a while to release the Hydrogen sulfide gas.