Hi Jens,
I use(d) Kirby 2 on some smaller client projects over the last year and was/am satisfied with it. I had a look at Grav out of curiosity only, not because I was really missing a feature in Kirby at this point in time.
I then migrated a site from Kirby 2.x to Grav 1.0 over a weekend and liked the process and results. What I like about Grav is the backend, the "engine" that is closer to the PHP I deal with in other bigger projects. So I felt more home from the start. I could say it's closer to current PHP programming best practices, but that's somewhat subjective I know.
So, one reason to choose Grav over Kirby is that I'm using tools I already use in other PHP projects (TWIG, Composer, CLI, Symfony Framework goodies).
I also like the idea to extend the Dashboard in Grav Admin with plugins, something that is not easy or impossible to do in Kirby without hacking Kirby Panel.
What got me confused at the beginning was the way Grav uses it's YAML Blueprints to create custom content types, configuration and define themes or plugins. After reading the docs and chec king sample code it clicked and felt really nice and rewarding. As a side note, Blueprints will be getting even better with the Grav 1.1 release later in April 2016.
What I'm missing in Grav so far is the media management of Kirby. I loved that I was able to edit media files in Kirby Panel, something not possible in Grav Admin 1.0. This problem seams to be dealt with in Grav Admin 1.1 (Pro) though. So with this problem out of the way, and the upcoming improvements, I think I will stick with Grav from now on.
I also checked Statamic 2 based on Laravel 5.1 and with Vue.js for their Control Panel, but was not impressed about the speed, something I really like about Grav too. It's really fast out of the box!
If you have any more questions, feel free to ask and we'll do our best to answer them for you. :)
Mike