It's great to hear that you're interested in studying at Manchester Met!
Have you visited both unis on their open days? That's usually a great way to help decide where to apply as you can get a feel for the campus and their facilities🙂 Another good way to compare the courses is to have a look at both uni websites and compare the modules, entry requirements and any accreditations the courses might have.
Choosing which uni to apply for can be difficult as there's so many to choose from. Sometimes it's nice to speak with current students to get their point of view. You can speak to Man Met students here (Cardiff Met will likely have similar functions, too) to ask them any questions relating to student life both at the uni and in Manchester.