Thanks for your interest in GNOME Boxes. I’m the maintainer. 
You could try to install the spice guest agent manually from https://www.spice-space.org/download.html in the guest.
Answering your other question: both virt-manager and gnome-boxes are frontends for libvirt+qemu.
Virt-manager is designed for users who know what they are doing and have entire responsibility of making their guest work. Boxes is for the user that just wants to run an OS and doesn’t want/need to understand operating system internals, drivers, settings, and all.
Boxes is part of GNOME but I would also consider it a Fedora project. I work for Red Hat fulltime in GNOME Boxes, in doing so, I target Fedora during the development and I also maintain the Boxes RPM package.