You will find much more qualified replies from other people on this forum. IMHO there are many reasons why you should go with the 840F.
Everything else being equal, You will get much more versatility in what you can achieve (and thus offer to consumers) having a wider temperature range.
You can always order some good pizza stone but you cannot do certain things at 570 (for example no NY elite, and certainly no Neapolitan style, panuozzi etc.).