PoikenzI think you mean 3 piece (3 buckle) vs 2 piece or overlap (4 buckle) boots. The selling point isn't the amount of buckles but the flex that a 3-piece boot creates.It is suppose to be more progressive, so when you lean in it gradually gets stiffer, a two piece is stiff the second you lean into it. Ankle hinge placement is also different and can feel a bit different. Really though, it's all about personal preference and what fits better. No matter what boot you get don't feel like you are missing out on a better boot design because they both have their pros and cons.
Definitely right that fit comes first. Concerning the flex pattern discussion- both boot designs can be progressive, both boot designs can be more linear. Having a boot that is 100% stiff from the moment you start to flex it does no one any good, so even the stiffest 2-piece boot needs to be progressive. Racers are the first to complain about anything being the slightest bit incorrect, so even these guys know the importance of having a progressively flexing boot.
Depending on the construction, flex curves of 2-piece ski boots can look somewhat similar to mountain bike or motorsports suspension curves. It all depends on the hinge point locations, wall thickness, wall geometry, and material selection. A boot company can make a 2-piece boot with a very linear flex pattern (which would be super weird and ski like shit), an ultra progressive flex pattern with a steep ramp up, or a nicely smooth progressive flex pattern.
Ultimately, the actual usable range that a cuff moves during skiing is not a lot. So if you were to plot out some 3-piece boots and some 2-piece boots and examine their flex pattern in said usable range, you would see that they can be identical.