Mercury is much closer to the Sun, so that whatever atmosphere it has exists as a thin layer of ions. Venus has a thick cloud cover that covers the whole planet and is almost entirely carbon dioxide.
Venus is even hotter (460 °C) than Mercury's sun-facing side, because the atmosphere traps solar heat. When one side of Mercury is hot, the other is in a deep freeze.
Venus is almost a twin of the Earth in size, while Mercury is the smallest of the 8 major planets, less than half the size of Venus.
Venus spins "backward" compared to Mercury and the other planets. It takes 243 days to make one clockwise turn
Mercury rotates more quickly, but has a longer solar day than Venus. (It rotates once every 59 Earth days, but its year is only 88 days, so any location will be in sunlight for 88 Earth Days and then night for 88 Earth days. Venus's solar day is about 116 Earth days.
There is ice on Mercury, in deep craters near the poles. Venus is uniformly dry and hot, with almost no water vapor, and no liquid water or ice.
Mercury has a weak but definite magnetic field. If Venus has one, it is practically undetectable.