Apples and oranges. The Heat is a stereo distortion unit, filter, EQ, while the WA-76 is a mono compressor. While the Heat can provide some pseudo-compression using its envelope follower, it’s not design to be a true compressor. The WA-76 only compresses/limits and isn’t meant to be a distortion unit. And you’ll need two of the Warm Audios if you want to compress stereo tracks.
Having said that, I have both and both excel at their intended tasks. If I could only have one, my decision would be based on what specific needs I had. If I were recording vocals or, say, electric bass guitar, I’d go for the WA-76. However, if I needed something to add an analog edge or full on fuzz, I’d grab the Heat, particularly if I wanted something to add some grit to stereo mixes or drums.