I already answered over on the Grav Forum and tried to shed some light on things I like about Grav.
In general I want to say that both systems are really nice to work with. That’s my subjective opinion based on having built several sites in both systems, Kirby 2.x and Grav 1.x, even migrating/porting on from Kirby to Grav. ;). I never made it into the guts of both systems, but used lots of the functionality. I tweaked the systems to my liking with blueprints/skeletons, custom themes, fields, plugins and more, to cater for my client needs.
At the end of the day, I’m happy to have both systems as options in my toolbelt. I used them as the tools they are, not because of marketing or fellow developers/desingers or herds say so. One project or client feels better with Kirby, while another has a need that I can better combine with Grav. Both systems are actively developed.
I must confess that Kirby looks more polished and clean, compared with Grav Admin 1.0, but it’s something that will change with Grav Admin 1.1 and even more with Pro.
Also, the Grav website and documentation need improvement, but that’s something each project needs to tackle and the Grav team is aware of that too. As far as I can judge, both teams are really responsive when it comes to issue handling and improvements.
Both have a solid vision and concept for their systems. Grav being the “youngster” gets a ton of “please implement this and that” to make Grav look and behave like ABC or XYZ. While Kirby has already established itself and speaks towards a certain kind of developer, designer and client. Grav has to find this niche/position.
It’s true that the Grav team wants to “sell” their Gantry Theme Framework. Yes, there is already a Plugin for Grav, but hey, you don’t have to use it and the same is true for the paid Admin Pro plugin, once it’s released.
And don’t forget systems like Statamic 2 that in the end of March 2016 or a Pagekit 1.0 in April. All available to help us bring the internet to life and help our clients and us to spread their message and visions. As long as there is fair competition I’m pretty sure, no matter what system you use, you will see improvements over time, and if you don’t see them, just migrate to another tool. It’s not rocket science isn’t it? We are not talking Typo3 or Magento here right?!