I see that it's been a while since you have shared this post. I'm wondering if you have reached some clarity with it?
Now whether you would like to pursue a full-time degree course or a diploma, might depend on some factors - such as, how you are looking at the future prospects of this for yourself. Are you looking at interior design as your primary career pursuit, or is it more of a creative interest for you (something you would like to pursue alongside another major career pursuit).
Secondly, regarding the duration of diploma (if that's what you choose), maybe you could try and look at the content of the courses and the difference between what each of it offers? Then if you feel that what is offered in the one year diploma is good enough, you could choose that. Otherwise, if you find that there are some important learning experiences and course content in the two year diploma course that you want to engage in, then you could choose that accordingly.
I hope this gives you some direction. If you would like more clarity and focused attention to figure some of this out further, you could get in touch with one of the career experts on the platforms for a session.
Oh and yes, it's good to see that you are in touch with your interest areas and are giving a serious thought about how to pursue the same.
All the best!