The main difference between the two is that UMTS is known to be a type of cellular technology while the WCDMA is known to be a type of air interface. The WCDMA is normally used in order to communicate with other devices. UMTS means Universal Mobile Telecommunications System. This is the newer and more modern version of the GSM networks that used to be rampant a few decades ago.
Some may assume that they are the same but the WCDMA is normally used to make sure that the signal will become more efficient and that the signal will be passed on properly to the different mobile network operators. This will ensure that people can communicate properly.
You may generally see UMTS as a type of cellular technology. WCDMA, on the other hand, is a type of air interface, which means that it will be beneficial in allowing different gadgets and technologies to interact with each other appropriately.
This means that if you are using a gadget and you want to communicate with another person with another gadget, the WCDMA can be beneficial. The UMTS is known to be the upgraded version of the GMS system which used to be popular a couple of decades ago. Both of these are known to improve the way that people communicate with each other, but they should not be mistaken to be the same thing.