Both translations attempt to weave the interpretation of rashi into the text so what you get is not a word-for-word translation of the hebrew text of the torah. Not to say that it's ideal to interpret the torah literally, but what you get is a watered-down, one-sided view of the torah and rashi's interpretation according to the view of the translator which i consider to be a downside. The gutnick chumash has a box with explanations of different passages, terms, or ideas presented in the torah according to various commentators which i think is kind of cool. Both have a running commentary based on the writings of the lubavitcher rebbe. As mentioned in the other answer they're both pretty similar, but kehot is the official publishing house of chabad. If i had to pick one I'd probably pick the gutnick but i would honestly find a different chumash if you wanted a more accurate or less biased translation. Of course all translations are biased but in this case it's very obvious and so fundamental to the translation it's hard to work around.