I think you'll get better answers with more information: cruising grounds, size of
boat, and lengths of chain you're considering.
Some will say that you want all chain, regardless. At a minimum I want chain that is sufficiently strong and long enough to prevent chafe on the bottom,. For my 11T
sloop I use 100 ft of 5/16" G40 chain (ie, 30M of 8mm). I would carry more chain (or all chain) in the tropics or high latitudes, but our
anchorages are mostly mud, gravel, and weed-covered rock.