I plan on running a horror-campaign, but which one is the best and why?
Strictly by the book, Ravenloft emulates Gothic Horror. Think
Dracula. Lots of vampires, werewolves, and animated corpses, lots of curses, lots of dark and stormy nights -- all mixed with D&D: Clerics, Wizards, Paladins, etc.
Call of Cthulhu presents rules for modern investigators in the modern world (1800's through today). The monsters aren't the classic monster-movie monsters but misshapen tentacled abominations from beyond, the kind of thing you'd find in H.P. Lovecraft's writing.
Ravenloft presents rules for horror checks; if you see something scary, it might scare you. Call of Cthulhu presents rules for insanity; you keep a running tally of your Sanity score, and unearthly horror might permanently scar your psyche.
And do they both have the same "frightening-potential"?
In many ways, I find Call of Cthulhu scarier. The investigators aren't generally great warriors, and even if they were, the monsters aren't carefully scaled to their power level. (Ravenloft has a bit of that too, just not as extreme.)
The investigators also aren't great wizards, and -- my favorite part -- magic comes with a price. No one automatically knows spells, and learning a spell costs Sanity. Casting it costs even more Sanity, along with a couple points of Ability Drain (e.g. Str drain or Wis drain). There are no free spells per day.