Slashers are way scarier for me than the supernatural. A human psycho stalking and trying to kill you is more realistic than supernatural with the fake jump scares and ridiculous plot twists.
It's horrifying thinking about what human beings are capable of. Although saying that Ringu and The Grudge, Japanese version, scared me plenty the first time I saw them. So I suppose it depends on how well they're made as to how effective they are.
A lot of slasher films can, in fact, be psychological, they aren't necessarily at odds with each other. 2012's Maniac is a fantastic example of that (the original as well, but I prefer the remake).