The meanings of these two acronyms are different. SFTP means SSH File Transfer Protocol, while FTPS means FTP over SSL or FTP Secure. FTPS has been created solely to add more security mechanisms that may be needed to make things more secure. SFTP is an extension that has been created for SSH.
SSH is already secure, so what the STFP can do is to ensure that file transfer is going to be easier to do. SFTP will only use one channel so that the proper data can be transmitted. FTPS would need to use another channel that will be used for the data. FTPS is more popular between the two, but those who would like to use something that is more secure can opt for SFTP.