My understanding is Faithful is the colour the sensor sees with no adjustments made to compensate for any tint or bias the sensor might have. Neutral takes into account the sensor's different sensitivities and adjusts the colour to try to match what the human eye sees. I think the idea is Faithful is for people who would rather fix the camera's colour themselves and Neutral is for people who are happy to trust Canon to judge the basic adjustments for them.
Either way, the way Lightroom handles the colour profiles is different to the in-camera profiles, and so for judging how a raw file will look the Neutral profile is more accurate to the file you'll actually be working with. I've checked this against correction profiles made with a colorchecker and it was consistent under tungsten, fluorescent, and day light. Lightroom's Neutral is closer to what a colorchecker profile gives you in hue, saturation, and contrast, while Lightroom's Faithful has a little more contrast between green hues and a little less between skin tones compared to what a colorchecker profile makes. Lightroom's Neutral profile for Canon raw is really neutral.
But it's definitely the smallest difference and I only make the distinction for jobs where I've been specifically asked to deliver a really even, accurate file. I think in most cases you're best off judging colour and contrast on a photo-by-photo basis.