At first I would look at the window film itself because there are huge differences. Most common films have bad heat/infrared reflection or blocking they are just tinted, you don't want that!
Then there are those with a metalized layer which have much more overall heat retention and sometimes reflective finish which makes it harder to see the interior with less overall tint (better vision from inside) but can have negative impact on GPS performance and other ota services.
Last but not least, the films with embedded ceramic particles which creates the best overall results and usually the best quality independent from the specific manufacturer. Famous premium manufacturer is Llumar.
I personally used Beluga285 from Höllwarth Folientechnik which is 85% tint ceramic film, installed it myself and quite happy with it. They also offer installation at their workshop but I would check the distance...
Link:
Höllwarth Folientechnik produziert und montiert selbstklebende Folien zur Verdunklung von Autoscheiben. Bestellen Sie im Online-Shop oder kommen Sie vorbei.
www.autoglas-scheibentoenen.de
This is an installer for Llumar in Frankfurt, no personal experience but heard that they are really professionals
Entdecken Sie bei Folienpartner eine breite Auswahl an Folie für Industrie und Handwerk. Maßgeschneiderte Lösungen und erstklassiger Service.
www.folienpartner.com