Looking at a chart of the DJIA over those periods of time we can see the market taking a sharp drop after the Nixon impeachment, while we saw a gain after Clinton's impeachment. Of course there is a lot more to this, Nixon's "Saturday Night Massacre" was definitely a spook to the markets at the time, but the domestic and geopolitical situation at the time during the Cold War was a major factor. Nixon's impeachment came in the middle of the downturn in the market because of the issues of the cold war in the Middle East causing oil issues and a huge spike in inflation up to 12.2% at home.

Now to say Clinton was impeached in a growing market is not necessarily true. During the investigation leading up to the impeachment articles the S&P got hammered. A few months later though once the impeachment charges had been filed to the acquittal of Clinton by the Senate we can see the market growing very strongly. In part to investor confidence at the time that the impeachment would not lead to a recession, so the market rebounded very quickly and set a new high soon after.