Just to add some more thoughts....... It also depends on the style of car you're building. Traditional cars look better with 4 piece, smoothie cars look better with 3. The 3 piece is harder in part because it's got less "movement" to use in aligning.

The key with a glass car is getting the body as squared up as possible on the frame. Given that some (all?) of them aren't all that square to start with you end up compromising some. You'll learn to fit everything loosely (a dozen times ain't unusual if you're a picky sort) to get it all aligned the way you want it.

As for the Hagan, unless he's changed how he does it, he sends you pattern making material with instructions. You fit and cut to your car, so, in theory, the fit should be "perfect". The reality is that it all depends on how accurate you can be. And then, even though you pre-build the car, carefully shimming and fitting each piece, then blow it apart to paint, even though you may have religiously marked every shim and mounting point, you'll still be doing tweeks on "final" reassembly. But that's all part of the fun of building from the ground up!