The main difference between Bibim Guksu and Bibim Naengmyeon is the type of noodle that’s used.
Bibim Guksu refers to thin, wheat noodles like Somyeon/Somen. The texture is soft and a little al dente. The thin noodles twist and wrap around each other for a pleasurable, soft bite of noodles.
Bibim Naengmyeon refers to noodles made from buckwheat, arrowroot, and/or sweet potato starch. Long and darker in color, Naengmyeon noodles have more stretch and chew. The texture is not as soft but pleasantly chewy and a little bit tough.