I just actually tried both of them out of idle curiosity.
They have that same sound-layers thing, sure. You move slowly in both, but that's a horror game convention (and you can't run nearly as far in Sanatorium as in Slender). No face flashes across your screen in Sanatorium, you just see the slenderman standiong where he was when you died, no closeup of his face (or I did anyway, I can't stand that Pewdiepie guy long enough to watch his videos). And any similarities between the way slenderman is handled is something that I can't really comment on as I'm not too familiar with the character, but is static/interferance something that's associated with him? I'm pretty sure Sanatorium isn't even made in the same engine as Slender, but I'm admittedly not too knowledgable about that sort of thing.
And as far as I can tell, the Sanotorium game isn't trying to be Slender. They've obviously put some effort into making their own thing while creating an omage to Slender. Their game comes with a "Read This First!" File that gives full credit to the original Slender game and its creator, and a link to his website. They aren't trying to steal credit, or pretend this was their idea. They seem pretty upfront about where they got their insipration. I'm fairly certain they make mention of the fact taht you collect 8 notes in their game because that's what you did in Slender and they wanted this to be a continuation of it? Something like that.
The main point I'm making here is that Sanotorium existing in no way detracts from Slender. Slender is a really short game based on a memetic character created on SA (I think, maybe 4chan?) who noone owns. The whole concept of Slenderman was a success because everyone could add to it. I think it started as an image contest or something. Pretty much everything in both games aside from that character is extremely derivative of other works in the genre, which isn't a bad thing at all. But it kind of seems off that a guy who made a game on the backs of other peoples works can then turn around and decry someone else doing the same (in a completely respectfull manner) off the back of his work.
Neither of them are even selling their respective games. So the whole thing basically comes down to "who made the atmospheric game where you walk around and a monster appears behind you" first. And that person is whoever made the SCP: Containment Breach game. (There's probobly loads of games that basically do the same thing before SCP: CB too.. But again, effort).
Also, now that I've played both games:
These are both games in the loosest definition of the word. They're more like interactive art exhibits or toys. Gameplay consists entirely of walking around and looking for notes. It's atmosphere to the exclusion of pretty much everything else. I actually preferred Sanatorium over Slender solely beacuse the indoor environment and more-usefull flashlight made the act of walking less frustrating.
I may be a bit biased against this, since I've spent time studying horror movies in depth, but these seem one step above a string of jump scares to me. They don't scare me at all, and it baffles me how people can play them and find them terrifying. And while I know they're both in beta, or at least Slender is, they really need more than almost aimless wandering to drive what's going on. I also can't help feeling that they're horribly wasting the medium of video games in their concepts. All of the atmosphere and tension is cinematic, by which I mean essentially that it's all audio/visual cues that movie audiences have been trained over the years to associate with fear. There's nothing like the SCP: Containment Breach blinking mechanic, which for lack of a better word is video-game-atic (whatever the video game equivalent of cinematic is) and generates fear and tension through the gameplay mechanics.
They both reminded me of a haunted house ride I went on when I was a child. You walk around, things pop up and go boo, then you walk out. I mean it's very cool that more and more games are being made and distributed for free by small teams like this, I just wish there was some substance beyond the ominous sound and spooky character model.