Bitcoin-core is currently about 97% of bitcoin nodes.

Geth is about 80% of ethereum execution layer nodes.

But the bigger difference is cultural - alternative clients are a core part of ethereum's development culture and is actively supported and nurtured. This is complete with social campaigns to get folks to switch away from majority clients to prevent situations where a bug in one client could cause big issues in the network. The same kind of environment is not present in bitcoin land.