i think for one, there are actually more options with regard to FULL kits
Sc'er kits include procharger,vortech,powerworks,and jackson racing, and they are all complete and relatively easy to install
turbo kits include focussport (i dont think they even make a zetec one), xios (huge price tag), focus-power, c-f-m, and there is the new topspeed kit.
however most of them are not complete off the shelf and include a slightly more involving install.
PLus most superchagers (especially used ones) can be had at a discount compared to a complete turbo kit.
thats the way i see it anyway