Rubico courts play quite a bit faster. I've played junior tournaments on both surfaces. Sometimes on a rubico court, it can seem a bit slippery and one has more difficulty coming to a complete stop and changing directions. You tend to "slide around" on rubico a bit more than red clay and it's definitely faster, with the balls bouncing lower. One notices the finer particles contained on a red clay court, versus the "pebbles" on a rubico court. A really nice rubico court is a lot of fun to play on, especially in the heat of a Houston summer! Yet, Red Clay feels a lot better to me. When you slide, the clay tends to sort of "keep you up better", so one can really run around pretty fast with good traction, given that you'll have to slide properly as you're moving laterally and forwards/backwards on the court.
See this excerpt on wikipedia describing rubico courts:
Green clay, Har-Tru or "American" clay, is similar to red clay, the differences being that it is slightly harder and faster. Green clay is packed to make the subsurface. It is then covered with a topping. These clay courts are found primarily in the Eastern and Southern parts of the United States, but are also located in all 50 states. In parts of the gulf coast region of the Southeast, green clay courts are often referred to as "rubico." There are two major WTA tournaments played on green Har-Tru clay courts; the Family Circle Cup in Charleston, SC and the MPS Group Championships in Ponte Vedra Beach, FL, a new event in 2009.