Hi there
We have a slate hearth after previously having had granite.
Would definitely recommend granite. Slate appears to be very soft and shows all marks. Granite showed nothing and was great.
We have a gas file but you have to move the front of the grate to turn it on. Very easy to scratch the slate. Dust also collects on the slate at a much faster rate to granite or maybe it is that you can see it on the slate before the granite.
If you do go slate, best way to keep it clean is baby oil. Messy job though.