Hey...
A diploma course in Interior design is only 1 -2 years long, however a degree course is more detailed and is outlined in 3 -4 year courses.
The diploma course in any field and not just design is no professional form you do because obviously one cant cover the complete teachings and learnings of the professional degree course. Its like having an idea about the stream but not knowing the details of the same. Diploma in interior designing gets you the same idea about how to deal with the stuff but it wont be easy for your mind to design efficiently as you couldn't open up your brain to think and apply your learnings to your work at a major extent.

So detailed learning will be helpful..
Choose accordingly.
All the best.
Hope this helps you!
Regards!!