There are advantages and disadvantages to both types of system. The salesperson may tell you that welded systems is stronger and more rigid than a bolted system which may be true, but don’t forget that a bolted system has been designed as such so makes allowance for the method of construction.
Bolted systems: With a bolted racking system you have far greater flexibility when repairs are needed. A bolted racking system is like a big Meccano set and you can usually remove and replace any individual component.
Welded systems: With a welded frame you are severely limited when repairs are needed. SEMA guidelines are clear that damaged components must be replaced so what do you do? Manufacturers of some welded systems do offer a repair kit for damaged frame bracing and as a manufacturer approved process this is considered acceptable if correctly fitted. Some welded systems also offer the option of a sacrificial front leg whereby the lower front section can be unbolted and replaced. Any damage elsewhere on the system means a whole new frame.
In terms of practicality a bolted system does offer greater flexibility for repairs.