Weapons grade steels corrode. Period. This is true whether it is high carbon steel or whether it is stainless steel. If your weapon has a stainless steel that will not rust, that steel is too soft for use as a true weapon.
The various Wing Chun lineages have specific types of butterfly swords (Baat Jam Do, or Bart Jarm Do) they require you to buy. There is no right or wrong type or style of sword. Most of the forms and swords types evolved over time with Wing Chun. The swords started off as non-lethal weapons, moved to very lethal, and exist as a hybrid today. What your teacher requests you to buy can tell you a lot about the era of the sword’s evolution he/she is teaching.