They're set for different purposes. On the Budgie ISO, the default branding is as follows:
- Document fonts: Noto Sans 10
- Interfaces font: Noto Sans 10
- Monospace font (such as when an application explicitly calls for whatever is set as monospace-font): Hack 10
For GNOME, I've opted for Clear Sans and Hack.
Then it seems to be that priority is Hack. Hack is a redesigned Bitstream Vera and DejaVu projects (approx. this should be... pairing). BTW, I love Solus Budgie x) more better than Solus GNOME for me.