I own modular version of Owl and standalone Magus. It's a slight upgrade in performance, but huge update in capabilities due to having:
- way more CV IO (Owl doesn't have CV output channels, only 2 audio can be used for this task)
- OLED display - it's no longer a black box when your patch runs, you can output debug info or visualize stuff
- USB host
Also, according to official info posted here, current version of Owl won't be produced due to lack of availability of chip it uses, but there would be a replacement with newer hardware eventually. So Magus is not intended to be a direct replacement.