John Sides talks with Julio Suarez, who has been researching the topic:
Recently they don’t look much different at all. When you take the average member appointed by a Democratic president and the average member appointed by a Republican president, you observe what you expect: Since 1936, Democratic appointees are slightly more dovish than Republican appointees, although the difference is not statistically significant. The average score for a Democratic appointee is -0.22 vs. -0.08 for a Republican appointee.
There is more here, including a graph and a link to the underlying data.