I respect all opinions but this, is a stupid debate lol. I mean, Slate who? hahaha...
Waves are the inventors of the audio plugin as we know it 33 yrs ago and never stopped renewing their catalogue ever since. Nuff said.
Cheers

 

Gotta agree with Nuffy: Waves plugins are rock solid. Overall useful tools and lots of them. If i need analog flavor i would not look either for Waves and Slate. I'd Rather use Nebula, PSP and PA/FAL. My wife Buffy also like Waves plugins, even tho she's not an expert. But still make some fire EDM tracks with Waves plugins. Nuff Said!!!

 

If I absolutely had to pick, it'd be waves. Slate makes some decent compressors, but as far as everything else they make, I don't find myself reaching for it as much. Just RBass alone from Waves makes them the choice for me, cause I use that on pretty much everything I make ever! But I might add that I make hip-hop/trap, so if you're making something like Rock or something with more of a live instrument heavy sound, you might reach for something else. I wouldn't know.

Also, I do think limiting yourself to one or the other between these two is not the way to think about it at all though. Just my two cents.