I think github and stackoverflow are both good indicators of how people use their spare time (and the quality of work they expect from themselves). It shouldn't be used blindly or out of context, however I feel both can be a powerful addition to a talent-judging heuristic.

To comment on the actual question, i.e. which do I value more, I'd say github. This probably comes from my bias of valuing execution slightly more than domain knowledge (and understanding executors better than domain experts).

Here's an example to illustrate why I believe this way. Person 'a' may write an instruction manual for some appliance -- this may take considerable skills (it is unclear whether they borrowed the instructions or composed them entirely themselves). How do we know if the instructions are accurate without first implementing them? Either we or someone else, let's say person 'b', must execute on said instructions and have the knowledge to know whether the build was a success. By this, I do not make the claim that everyone on github who builds things writes good code (or even understands if they've assembled the parts correctly). I have, however, experienced that a good github profile (on average) provides me with more information than a stackoverflow post. Do they contribute with others, do they comment their code, do they catch bugs, do they write efficient code, how regularly do they contribute? Granted you can obtain some of this information from stackoverflow, but this leads me to a question of my own -- why choose just one?