If you want pixel perfect output (i.e. 99.99% of the time) you should have ppm the same on both.
Changing the ppm on the camera will show more or less of the scene, eg. 2x or 0.5x ppm = will show twice or half the amount of information. The sprites themselves are the same physical size.
If you change the ppm of the sprite collection, you get the same result, but this time the camera sees the same amount of stuff, but the sprites are 2x or 0.5x the size as they were before.
Make sense so far?
Now, if you physically change the size of the sprite, eg. from 128x128 -> 64x64, the sprite will naturally appear 1/2 the size as it was. Imagine 2 sprites positioned side by side, once you resized it, you'll see a gap between them. 2 choices - reposition the 2nd sprite, or... scale the sprites up 2x. #1 involves setting up your scene again in most cases, however, luckily its easy to do #2 - you just change the ppm on the collection to compensate. In this example, changing the ppm on the sprite collection and not the camera will result in exactly the same scene as before, but with less detail. If you then want to adjust your camera to match thats up to you - thats the easy bit.