|I can tell you about my Powertrix spoiler from about 4 years ago.
Lots & lots of sanding the underneath mounting surface to obtain the best fit. This of course was done prior to painting. Many trial and error placements on the hatch to get the best condition.
My advice- which ever one you get -don't depend on the few fasteners to pull it down into place. The double sided foam tape is going to provide most of the mounting strength.