Let's be real, this is a tough call. But forced to choose, I'd take the AI Overview mentions.
The reason is quality and intent. Being cited by the AI overview drives direct, authoritative traffic that is currently unmatched. The ROI from those 5 mentions would likely dwarf the 10 guest posts.
However, a sophisticated strategy recognizes that this is a false binary. Systematic guest posting is one of the most reliable methods to build the topical authority and backlink that the AI algorithm rewards. The guest posts are, in essence, what qualifies you for the AI overview.
So, in summary: Target the AI overview as the outcome, but invest in guest posting as the foundational tactic to achieve it.