We have chosen quartz with a mirror fleck because it is quite modern looking. Quartz is also more resistant to chemicals and spills, and you can use kitchen cleaners on it (I was told that you were better off cleaning granite with more gentle cleaners or just soap and water). I know nothing about radon, sorry.
I have both quartz and granite in my kitchen for the last three years. The quartz is a big slab on a central island in red/mirrored which looks v. glam!
Granite gets darker over time and is more absorbent of water/stains/grease. The quartz has been v. resilient but recently put two v hot pans directly on it and it has left two faint rings as if the finish has been affected.
Granite is pretty robust, I have had granite for 4 years and it has never marked or split and it gets a bit of rought treatment and the odd hot pan on it.
Granite is great - we have black granite for our counter tops and you can put any hot pans or any sort of cleaner on it and it remains in the same condition. I've used Cif with bleach in it and I've used an organic cleaner all to the same effect.
The only thing that bugs me is the need to 'shine it up' - if you wipe it with a damp cloth it looks dull until you buff it a bit.