Saab,
Interesting observation about the people who do both.
However I think you've answered it in your second paragraph. The nitty gritty (e.g. scan engine etc.) is likely to pretty much be a mirror copy of the free version, as is the case with Linux and it's kernel and core components.
What they will probably tag on for your money are a few extra bells and whistles, plus access to paid for technical support. But as you say, performance and update frequency are unlikely to change.
It's the same thing that's happening with Linux. Most of the platform very similar between free and paid. What you pay the vendors $$$ for is the 24x7 tech support and vendor developed business class features (e.g. clustering, management etc.).