Before you login, there is an icon on the login screen for "Session"; select your desired desktop then and login.
Mint will ask if you want to make it your default; answer "yes."
There is a way to select your terminal from the command line; it will involve creating an .xinitrc file and several other modifications (there may be other ways, less orthodox ways that I'm not familiar with). I doubt you want to go there. If you are interested, you can read about .xinitrc on the Arch wiki
; it's oriented to Arch, but it's a good reference.