> Smaller size at runtime (uses less memory).

Yours is smaller (in terms of sizeof), because std::function employs small-buffer optimization (SBO). That is if the user data fits into a specific size, then it's stored inline the std::function, instead of getting heap allocated. Yours need heap allocation for the ones that take data.

Whether yours win or lose on using less memory heavily depends on your typical closure sizes.

> Faster at runtime

Benchmark, please.