I guess it depends on how you define "GUI", if you would choose 'Xvesa' as the X server, 'flwm_topside' as your windows manager, 'wbar' as your launch bar and also want to use the TC specific GUI applications (e.g. appbrowser') then (at least AFAIK) "MicroCore with GUI" equals TinyCore. BTW I believe the term 'MicorCore' is now a bit a of historical nature and should be rather replaced by 'Core'.
So, I would choose the "Core plus GUI" route if I want a different option (e.g. a different X server) and not have the "standard" TC component "lurking around" in my system. But for practical reasons I only ever download the TC ISO file as that represents for me the best choice (including the option to create my own 'Core' based remaster if I'd choose to do so).