Interesting dilema. They will both play, they both play well both positions.
Pro Karras @C:
1. If you put Karras to guard you change two positions instead of just one which is against Dante's main principle of least moving parts (unless you're forced to)
2. Thuney is rated among top LGs in the league and Thuney-Solder combo is glueing and improving.
Pro Thuney @C:
1. Center is “green dot“ position; arguably requires more experience,
Thuney has more experience on the line which probably puts him in better position for playcalling, be comfortable on hard count etc.
Id start w Karras and if there are communication problems change the two on the fly..