It's mostly cosmetic. Personally I like the more simple and streamlined effect created with having the
mod match the atomizer from top to bottom. While it does eliminate the top cap it also seems to make the switch on the bottom stick out a few threads longer. How much depends a lot on the particular atomizer mounted. For instance the fogger must have the longest 510 male connecter (I haven't measured) since it seems to have affected the bottom switch the most. However if a person were to shorten the brass screw that comes in the hybrid kit you could not only eliminate the extra length in the switch area but possible reduce the
mods height even a bit more. I have modified my switches for a very short and stiff throw so I do plan to fiddle with shortening the brass screw in the hybrid top cap a bit at some point.
I have never measured voltage drop on any of my mods. I did think that all three Nemesis hit a touch better with the hybrid kit but that could easily be wishful thinking on my part.
There is also the option of using this kit to connect your atomizer directly to the battery itself by eliminating some parts from the kit. However your atomizer must have a positive pin that will sitck out well past the 510 connection on the atomizer or a hard short will be created. Also it's worth considering that heat can be transferred to the battery if you use and atomizer like a dripper that gets really hot.
fast tech sells the kit for a few cents less that only includes the parts needed for a direct to battery atomizer connection.