Just my opinion from owning three motorhomes and from reading RV forums for the past 13 years, there is not enough difference in either to make chassis the basis for my decision on a coach.
That said, I believe we are in a era when quality is suffering on many fronts including the companies that make the parts that both FL and Spartan use to build the chassis. Ball joints, steering gear, rear ends, etc. are all made by other companies. You can have issues with either brand due to issues with the stuff they buy to make the chassis.