It's the length tha you use when you anchor. In a given situation, you are likely to use the same length of chain because of swinging room restrictions (other boats, etc.).

In that case, the heavier chain would provide somewhat higher security. However, I doubt that the difference between the two chains would be significant. And a longer length of the lighter chain might be handier in the right situation than a shorter length of heavier chain.

I would choose the lighter chain with a longer length. JMHO