With the greatest respect to the (old) factory - I don't know what the new one charges for a build, nor what the end result is like; buying a factory built car will get you a car of a known minimum standard build quality, that is safe, drivable and able to pass IVA. However, it will also be a car built as quickly as possible, with the least work required to meet those goals. It will not have additional finesse work, it will be built with care and passion for the marque, but it will not have the high level of attention to detail that just takes to long to achieve commercially, it will not - unless explicitly paid extra for, in fact have anything above and beyond the basics. That's fine, as long as you understand it.
But you know what you're getting.
A kit bought car on the other hand, (depending on age of course), if it's from the SVA/IVA era, then you know must have been built to a safe standard, sufficient to pass the (rigorous) test. Beyond that, it's down to the original builder. It is perfectly true, there are some absolutely shonky builds out there! But the average is probably what a factory build would look like given infinite time to refine and do things that bit better, whilst the above average builds take the car places the factory could only dream of, and would double or more the cost of the car, if they tried it commercially.
So you need to examine a car more closely, and get a feel for what to look for and how to spot a good one.
The thing that can throw a complete spanner in the works though, is that very few Westfield builds whether they're factory or not, remain static; a huge proportion of owners will modify and change the cars over the years, often starting immediately once IVA is out of the way. I've seen factory cars out there, where for one reason or another, not one trace of the original factory build still remains!