If you are referring to the worksmanship between the two, there is really no differences. At the end of the day, ID still hire contractor to do the carpentry, tiling etc for them. A contractor also does the same things. I feel that all the so called ID shops i had been through are not really design based. They just mainly copy and paste from a template or from previous projects. I went to a few ID shops before and most of their own projects are more or less similar.
But if you really engaged a design base ID, that is another story. The one i engaged only design for their customers. They don't really carry out the construction work. What you are paying is for their design. The design fees can range from 2-5k. These are the real interior designers. They intergrate space planning and furniture selection(very important) into the design work.
If you are comparing the pricing between the two, then there is a huge difference. The quote i got from the contractor is about 4-5k cheaper than the so-called ID. But here is the catch, you need to supply a design/drawing to the contractor. They can do very basic drawing though. Even with the design you give them (with dimensions) the end product might not be 100% of what you see on the 2-D drawing because he might not understand the design in full details (since he is not the one who draws it).
Most ID will include labour cost for haulage which can cost up to 1k or more, but the few contractors i went to don't charge you for haulage. With that, you already save 1k.
One suggestion i can give you from my own experience is to have the best of both world. I engage a contractor to do the non-design based work like tiling work, painting, hacking of wall, etc. Then i engage a designer to come up with a design and he help me to liase with his own contractors to execute the carpentry work.
All of these are from my own personal experience though. Hope you have a smooth renovation.
