You can set Complex Pro mode as the default Warp mode by entering the Ableton preferences, navigating to the "Record, Warp, Launch" menu, and selecting from the default Warp mode drop-down menu.  You may have noticed that "Complex Pro" is not an option here. I am not sure why that is, but luckily unless you're looking for extreme control of the transients of your sample, Complex mode should be fine. Additionally, it is a simple process to change the Warp mode of a given sample.