The diluted side has a rather dull, 'soaked in' look. You have to examine the seam closely to see that it was sealed.
The undiluted side is shiny, and is obvious from a distance that it's been sealed. As the other poster pointed out, it's very waterproof. It's also kind of 'grippy' or sticky in texture.
Overall, if the long term performance is equal, from now on I'll use it undiluted simply because it was simpler (no mixing, stirring, cleanup, etc). Just squeeze a fine bead along the seam, and smooth in with a wet finger.