I've been exclusively a fountain pen user for almost 40 years. One of the issues/frustrations that users encounter is how long a pen can be idle and still write immediately when the nib touches paper. In my experience, pens with screw on caps generally perform better in this regard than pens with snap-caps.
But, as Oliver Wendell Holmes once said, 'no generalization is worth a damn, including this one'. For example, I have a Platinum Preppy that I've updated by replacing the plastic body and cap with the aluminum body and cap from a Platinum Plaisir. The pen is loaded with Platinum's blue-black iron gall ink. It has a slip on cap, and it writes immediately every time I pick it up.
My sense is that the screw-on cap feature is one of the characteristics used to justify higher prices for both manufactured and kit pens. It is also the case that physical considerations means that screw cap pens generally are larger in diameter than snap-cap pens - threading takes space, and material has to be thicker to allow for threads which means the body of the pen be bulkier. So if you are looking a kit pens, kits that are built around 10mm tubes are almost always snap cap, while screw-cap pens require larger tubes.
One comment that you didn't ask about - my sense is that most pen kits that are designed around 10mm tubes do not have center rings (ie, a metal or plastic component that dresses up the open end of the cap). As result, when the cap is removed, one can see the end of the cap material (be it wood or plastic). Conversely, pens with threaded caps always have a center ring that dresses up that potentially raw end. Again, there are exceptions - I have a PSI 'Cigar Pen' 10mm FP that has a center ring with a snap cap, but I believe that's not common. I find that center ring to be more aesthetically pleasing than a raw end.
Finally, my experience in making FPs from kits is that snap caps are a PITA to assemble. The snap cap consists of a plastic receiver that fits inside the cap, and that screws onto a threaded stud at the bottom of the cap finial. On the kits that I've made, the female portion of the receiver didn't come threaded, and it was necessary to either find a metric tap to thread the plastic receiver to match the male stud, or else use the stud to cut threads in the female portion. Doing the latter with the receiver inside the cap is very difficult. The instructions typically suggest putting a Phillips screwdrive inside the receive to drive it onto the stud, but my experience with that approach has not been good. Eventually, I ground the tip off a sacrificial Phillips screwdriver to create a star-shaped driver with sharp corners that could grip the inside of the receiver. And by the way, those plastic receivers can break.
The other question you ask relates to posting. This is sort of like boxers versus briefs - some people choose to post, while others choose not to post. There are a few pens (such as some of the Kawecos) that are physically quite small are are designed with the expectation that the cap will be posted, but for pens of ordinary length, its purely a matter of user choice.
