Richard,
I believe the SPF chassis is much simpler to build. Square tubing chassis’ are easier to fabricate simply because the tubes do not need to be coped. It is also quicker and easier to weld square tubing because it is a straight line weld. Also, the SPF chassis is less complex and appears to have less parts welded onto it. Time is money!
Have no experience with the SPF cars, so I do not claim to know anything about performance, durability, quality, etc. I am simply stating my opinios based on my observations.
