Thought I’d bring a different perspective here. I’m not massively familiar with Grav, but I know some other systems very well that follow a similar approach to Grav.
I can’t talk about system differences (I’m new to both). But from the instant attraction to newcomers, Grav wins there. Not because it’s better (Kirby feels better when you spend time with both)., but because it sells itself better.
Grav is open source, free and community driven (though it is from a commercial theme provider). Kirby is open source, low cost but lacking community drive (I’ll explain below).
Grav has very accessible plugins, viewable and installable through the admin, Kirby depends on github and a third party site.
Grav has a very clear and accessible list of themes. Kirby relies on a commercial third party site.
Grav has a very cool looking admin panel that could impress a client. Kirby has a nice simple admin panel that is clear to use.
To be clear, for me if it was a choice between these two, Kirby would be my choice. But I bet that most visitors to both projects will initially choose Grav for the reasons above.
The downside to Kirby for me is the lack of encompassing community - the forum community is strong but the plugins are 3rd party controlled, themes is a commercial project. As someone that is used to be very involved, where will my support end up? For example, there’s less motivation to make free plugins or themes (which projects need nowadays) when they are controlled by commercially invested non-Kirby site.
If there is one thing Kirby needs to change, it is to contain it’s eco-system within the Kirby brand. Nothing wrong with commercial themes and plugins, but they should not be Kirby’s only resource.
If Grav ends up winning this ‘battle’, I bet it’ll be more about the eco-system and encompassing fully Grav controlled community than any of the features.