That depends on the hull. Would you rather have a typical solid glass hull that is 1/4" or 5/16" thick, or a cored hull with 3/8" balsa sandwiched between glass with a total thickness of two inches?
Why would you bother with a 3/8" core if the total thickness was 2"? Why would you compare that to a single thickness of 1/4 to 5/16"? The purpose of using a core is to space the two loading carrying surfaces further apart thus making a stiffer beam. It does get much stiffer, but it actually loses strength against a puncture. Core only makes sense if the core thickness is a large percentage of the total thickness.
Gary H. Lucas